History ThatOneBoss / Arknights

* ''Design of Strife'' features the final Diablo Sarkaz: Sverdsmeltr, Guardian Ember of the King. In his first form, he deal extremely strong Arts melee attack which will down any defenders in one or two hits, he also periodically use Suppression to incapacitate one Operator, and if your Operator retreats or defeated, they will be sealed until the Haunting Flames are destroyed. Despite that, you can take advantage of his Suppression to relocate your Operators without the need for redeployment time. Once his HP reach zero, that's when the troublesome part begin, he will use Payback to engulf everyone around him with Haunting Flames and releases Condensed Heat towards your Operator until he switches to his second form, where he will summon Reforged Retinues and continue releasing Condensed Heat. When his HP reaches 50% or all his Reforged Retinues are defeated, he will absorb all the remaining Reforged Retinues and recovers his HP and it became even worse, he now fires hard hitting AOE Arts attack while using Extinction on an Operator to incapacitate, which now spawns a stronger Condensed Heat that needed to be taken down before it instant-wipe your whole defenses and it will use Sunwielder's Sanctuary after Extinction to protect against Arts damage while given a massive boost in DEF and dealing Arts and Burn damage around himself.

** The Troupe Mouthpiece is the most unique boss in ''Arknights'', but that just means the player is more likely to get blindsided. Just to encounter the boss, like Gravestone from ''Fungimist'', the player will have to handicap themselves with one collectible (increases Operator DP cost by 2), which can't be mitigated until late in the run; of course, a [[ThatOneLevel secret level]] on the sixth floor is more than happy to end their run too. Then the boss level itself rears its head; from the get-go, it has a condition that halves natural DP generation, and boasts an entourage of DemonicSpiders like Derisive Audience, Scarlet Singers, and Theater Golems, the former of which can do massive Nervous Impairment damage. Meanwhile, the Troupe Mouthpiece himself is invincible at all times and can only be damaged by by eliminating the Host Assistants it periodically spawns to harass the player. The assistants initially appear to be flimsy and harmless, but just being in their proximity reduces Operators' attack power by 30% while dealing continuous [[ArmorPiercingAttack True damage]], and they have reduced aggro to direct Operator attention away from them, making them more durable than they look. With the way the map is designed, it's quite easy for operators to always be in their range, and any Operators that are carelessly placed near the middle will be swiftly undone by the Mouthpiece himself, who uses very powerful multi-target attacks that inflict extra Nerve damage. If the player takes too long eliminating the Assistants, the boss may decide to respawn them at full health as well, undoing some progress. Even after 10 Assistants have been killed to take down the Mouthpiece, he'll get back up and enter a second phase where he spawns all of the Assistants ''simultaneously'', potentially overwhelming defenses already occupied by the enemies on the map while forcing players to defeat them all over again; if the Assistants aren't all dead by the 50th enemy or so, the player better haul ass before the Mouthpiece waltzes right into the blue box for a -30 LP deduction. Let's just say the players will need at minimum one Elemental Healer, a powerful Vanguard and a lot of other Operators to throw at the Host's Assistants, all of which requires a copious amount of luck outside of just trying to encounter the stage itself.

* ''Design of Strife'' features the final Diablo Sarkaz: Sverdsmeltr, Guardian Ember of the King. In his first form, he deal extremely strong Arts melee attack which will down any defenders in one or two hits, he also periodically use Suppression to incapacitate one Operator, and if your Operator retreats or defeated, they will be sealed until the Haunting Flames are destroyed. Despite that, you can take advantage of his Suppression to relocate your Operators without the need for redeployment time. Once his HP reach zero, that's when the troublesome part begin, he will use Payback to engulf everyone around him with Haunting Flames and releases Condensed Heat towards your Operator until he switches to his second form, where he will summon Reforged Retinues and continue releasing Condensed Heat. When his HP reaches 50% or all his Reforged Retinues are defeated, he will absorb all the remaining Reforged Retinues and recovers his HP and it became even worse, he now fires hard hitting AOE Arts attack while using Extinction on an Operator to incapacitate, which now spawns a stronger Condensed Heat that needed to be taken down before it instant-wipe your whole defenses and it will use Sunwielder's Sanctuary after Extinction to protect from Arts damage while given a massive boost in DEF and dealing Arts and Burn damage around himself.

** The Playwright, added in the second major expansion, is just as hard if not harder than the first two secret bosses. His inability to be blocked is complemented by his insanely high durability, and his Arts attacks are simultaneously global-ranged, hard-hitting, and able to deal splash damage to adjacent units. He'll also periodically target all units with explosive Arts that can potentially wipe out entire defensive clusters from the overlapping explosions. However, his most dangerous move is the ability to periodically curse a target, ''permanently'' halving their attack speed and dealing Arts DamageOverTime [[PercentDamageAttack scaling with their maximum HP]]; if the target retreats while cursed, their redeployment time will be heavily increased and the Playwright will permanently seal the tile they're on a la [=FrostNova=], gradually reducing the space you have to deploy on. And just to add insult to injury, once he gets below half health, he'll [[TurnsRed enrage]] and reduce his skill cooldowns while significantly increasing not just his damage output, but his already high defenses (giving him ''2000 DEF and 90 RES'' without reductions), making it difficult to finish him off when he's taking ScratchDamage from most attacks and flattening anything that draws his aggro. All of this is bad enough, but not only does the Playwright spend much of his time loitering in isolated parts of the map while raining hell, he serves as suppressing fire for the DemonicSpiders on his level like the Winterwisp Blood Magisters, elite Originiutants, and Theater Golems, with his interference usually being enough to let them roll right over your crippled defense.

** Rarely, the Demon's Blade may be replaced with its Vagabond variant. Although his range is shorter and he moves even slower, his HP is even higher and the radius in which he spreads Dominion is ''tripled'', letting him douse the entire map in an instant. Fortunately, he does take a less direct path that is blocked by rubble to slow him down, giving the player more time to take advantage of his shorter range and shoot him from afar.

* [[TheJuggernaut Patriot]], ''good god''. Mere hours after his debut in the Chapter 7, he was already being hailed as the toughest boss in the entire game, and for good reason. Not only are his stats ''extremely'' high, he has a large number of skills to make any Doctor pull their hair out in rage.

** Izumik, Source of Ecology, takes things even further beyond any of the previous final bosses. His stats are even higher than the likes of the Last Knight or Ishar'mla, but he is instead an invincible FlunkyBoss that initially resorts to summoning ridiculous numbers of Offspring which have insane bulk and can transform into other enemies when blocked, all of which are hand-picked to be DemonicSpiders of the highest degree and buffed specifically for the stage. The player will almost certainly be forced to block them not just because of their aforementioned bulk, but the fact that letting enough of them reach Izumik himself will not only cause him to gain a ton of extra HP and ATK for his second phase, but also inflict massive Arts damage to all Operators on the field to further disrupt things; however, Izumik also moves frequently and specifically paths to intercept the swarms of jellies, making stopping them much harder than it looks. Once enough time passes (or worse, if he absorbs enough Offspring to completely fill his HP), things will get dicey as Izumik puts his gargantuan stats (which may be buffed further by his absorbed minions) to use by fighting back - his attacks do tremendous ''mixed'' damage that can instantly take out most units on higher Surging Waves levels, and every time enough time passes or he takes enough damage, he'll unleash a global Arts DamageOverTime and stun to let everything else waltz through your lines, if the damage over time and him attacking while they're down doesn't cause a TotalPartyKill to begin with, especially considering that the DOT scales with his ATK and thus can get massive buffs from his consumed minions or Surging Waves levels. Furthermore, like Ishar'mla, the player will have to gimp themselves early on by taking a collectible that ''instantly empties their Light and inflicts a Rejection;'' ''unlike'' Ishar'mla, it also requires that you ''get back'' that Light, as the item you get in the actual fourth ending encounter depends on the Light level at the time of acquisition. The strongest item can give you a real edge with its tremendous defensive buffs, but if your Light management isn't in top form, you'll have to settle for the considerably weaker normal item, or worse, the last one that ''debuffs'' your units instead. And all of this is ''at base'' - if you're on a high Surging Tides level, everything listed is exponentially harder, and you'll ''need'' that top-grade item, along with a boatload of other powerful collectibles, if you even want to stand a fighting chance against the sheer pressure on his boss stage.

** The Last Knight is a ridiculous MightyGlacier with '''100,000 HP, 4000 DEF and 70 RES''' at base and a mechanic designed to shut down brute force strategies relying on using units such as Surtr or Młynar to burst him down. Whenever he takes damage, he applies 2 seconds of Cold to the source - not per hit, ''per damage instance,'' meaning that units who deal multiple applications of damage per hit such as Mech-Accord Casters or Młynar using S3 are immediately Frozen, while units who attack too quickly will Freeze themselves as well. The only forms of DPS that can negate this are damage sources that count as separate from an Operator and can attack while the Operator is frozen, such as Goldenglow's detached S3 drones, Pozëmka's Typewriter, or W's mines. On top of that, he also gains a hefty 75% more ATK against Frozen targets, which even at moderate Surging Tides levels guarantees a OneHitKill on most Operators due to his substantial 2000 base ATK that is empowered by the difficulty modifiers. A saving grace is that there is a line of rubble in the Last Knight's path that he must break down before he reaches your melee Operators, during which time you must try to defeat him through his Cold debuff. After the arduous task of chipping his health down to 50% while your Operators are being repeatedly Frozen and having their attack speed reduced by Cold, however, things get worse as the Cold duration he applies is doubled to ''4 seconds'', all but guaranteeing that even slower attackers will get a second hit in while debuffed by Cold and become frozen. And while one may consider using Lee for his second Talent to negate the Freeze and block the Last Knight consistently - this can work to an extent, but because applying the Talent makes his next DP-drain cycle cost 5 DP instead (4 with his Module upgraded), he will eat your DP up ''fast'' without a Flagbearer supplying him with a constant stream of DP to keep his Talent going. If defeating him under these circumstances weren't difficult enough, he ''revives'' once defeated the first time (albeit only at 25% HP) and mounts his steed Rocinante, becoming ''unblockable'' and turning into a TimeLimitBoss that you must finish off before he reaches your base. Fortunately, he does not attack at this stage and moves at half his regular speed, but his first defeat also triggers the second wave of enemies, which includes Leithanian Psalmists, Remnant Principal Violinists and Retching Broodmothers that are assaulting your units with Arts damage (or Corrosion for the latter) while you are desperately trying to burst the Last Knight down. Fortunately, the Last Knight is vulnerable to status effects and debuffs, meaning applying Stuns, Sleep or Freeze are the best ways to stall him and delay his movement for as long as possible. On the highest difficulty levels, this is essentially required, as units such as Erato, Mr. Nothing (whose Unexpected Strike Talent has time to activate while he is Frozen) and Gnosis become the most valuable Operators to have against the boss due to their rapid application of status effects.

** Izu'mik, Fount of Ecology, takes things even further beyond any of the previous final bosses. His stats are even higher than the likes of the Last Knight or Ishar'mla, but he is instead an invincible FlunkyBoss that initially resorts to summoning ridiculous numbers of Offspring which have insane bulk and can transform into other enemies when blocked, all of which are hand-picked to be DemonicSpiders of the highest degree and buffed specifically for the stage. The player will almost certainly be forced to block them not just because of their aforementioned bulk, but the fact that letting enough of them reach Izu'mik himself will not only cause him to gain a ton of extra HP and ATK for his second phase, but also inflict massive Arts damage to all Operators on the field to further disrupt things; however, Izu'mik also moves frequently and specifically paths to intercept the swarms of jellies, making stopping them much harder than it looks. Once enough time passes (or worse, if he absorbs enough Offspring to completely fill his HP), things will get dicey as Izu'mik puts his gargantuan stats (which may be buffed further by his absorbed minions) to use by fighting back - his attacks do tremendous ''mixed'' damage that can instantly take out most units on higher Surging Waves levels, and every time enough time passes or he takes enough damage, he'll unleash a global Arts DamageOverTime and stun to let everything else waltz through your lines, if the damage over time and him attacking while they're down doesn't cause a TotalPartyKill to begin with, especially considering that the DOT scales with his ATK and thus can get massive buffs from his consumed minions or Surging Waves levels. Furthermore, like Ishar'mla, the player will have to gimp themselves early on by taking a collectible that ''instantly empties their Light and inflicts a Rejection;'' ''unlike'' Ishar'mla, it also requires that you ''get back'' that Light, as the item you get in the actual fourth ending encounter depends on the Light level at the time of acquisition. The strongest item can give you a real edge with its tremendous defensive buffs, but if your Light management isn't in top form, you'll have to settle for the considerably weaker normal item, or worse, the last one that ''debuffs'' your units instead. And all of this is ''at base'' - if you're on a high Surging Tides level, everything listed is exponentially harder, and you'll ''need'' that top-grade item, along with a boatload of other powerful collectibles, if you even want to stand a fighting chance against the sheer pressure on his boss stage.

* Mandragora from Chapter 9 can be a tremendous pain in the rear mostly due to her Stoneshield, which drastically reduces Physical and Arts damage taken, and can only be broken by dropping a Tattered Pillar on her. The Pillars in question are single-use and not always placed in convenient locations, making it very possible to permanently waste your chances to bring her down - not helping is that the levels she's fought in will often demand some of the pillars be used for rerouting or killing elite enemies, further limiting your chances at breaking the Stoneshield. Once you crack the shield, you have only 25 seconds to lay down the hurt on Mandragora before she grows it back, and she'll also instantly regain it if she takes enough damage at once, just to make bursting her harder. Offensively, she's also a hard hitter with alarmingly fast Arts attacks and Mandragora's Gaze, which deals massive continous damage while crippling the target's ASPD and potentially sealing the tile if the target is retreated under its effect. However, once you take out her first health bar, her second phase is the real kicker, as she'll start to ''fly'' while gaining a FlechetteStorm aura, ranged attacks, and the ability to manually bring down pillars on top of operators for a OneHitKill. The flight can be extremely damning as it'll make her unblockable and immune to non-ranged attacks, rendering her near impossible to stall or stop unless you can bring her down - and bringing her down is now harder since she can fight back at range and use the Pillars against you, wasting your chances of defeating her while disrupting your formation and making it even harder to fight back.[[note]]However, it's worth noting that her new ability to collapse Tattered Pillars can be an advantage, as she can be tricked into [[HoistByHisOwnPetard collapsing a Pillar onto herself.]] Additionally, Amiya's S3 is able to hit flying enemies and [[ArmorPiercingAttack deal True damage]] at the same time, which is very helpful against the second phase where Mandragora has Low-altitude hovering and does significant damage enough to dent most of the health bar, if not outright defeating her.[[/note]]
* The Damazti Cluster in Chapter 12 is a particularly tough boss to handle. To start off, the Damazti Cluster always starts the game disguised as a civilian, which means you cannot alpha strike them early with long range or helidrop Operators until you reveal the Damazti Cluster. However, once you do reveal the Damazti Cluster they will deal Corrosion damage to any Operator blocking them, eroding your blocker's DEF and when they hit 50% on their first lifebar, they summon a second copy of themselves that shares their HP and abilities and they both take reduced damage. When their first lifebar is depleted, they then start dealing massive AOE Arts damage which can wipe out your Operators if they are packed too close together. Finally upon hitting 50% on their second lifebar, the Damazti Cluster deals more AOE Arts damage and will target a random Operator (prioritizing the one with no allies in adjacent spaces), immediately force them to retreat and block their ability to redeploy, and then summon a ''third'' copy of themselves. Obviously, the challenge comes when the Damazti Cluster and its copies begin pushing down separate lanes, which will inevitably spread your forces thin trying to contain them. Even worse, the challenges maps will power up the Damazti Cluster such as allowing them to summon ''three'' copies instead of two, meaning you'd have to deal with four different Damazti Clusters simultaneously. Obviously, Therapist Medics, high DEF Defenders, Fast Redeploy Specialists, and AOE damage dealers are highly recommended.

** Al-Rafiq is a HumungousMecha with extreme durability, powerful attacks (including a ChargedAttack dealing splash damage), and the ability to self-destruct on death, dealing massive damage and a stun in a wide area to possibly let his allies through. After this, you have 30 seconds to deal roughly 100k damage to the pilot that just fell out of the machine, which is easier said than done when he's ''invisible'' and also has an AttackReflector that can instantly take out fast-attacking units. Failing to do so will result in the pilot repairing the mecha and ''reviving with full HP,'' turning the fight into a painful DPS check. On top of that, his army includes extremely tanky powerhouses such as elite Mudrock Zealots, Originiutants, and Maddened Vein Guardians, the latter of which are especially painful due to their own AttackReflector taking out units before they can even try to fight Al-Rafiq himself.

* The bosses in Reclamation Algorithm mode that pop up in Day 10 and march to your base are absolutely ''brutal'', with inflated stats and potentially deadly mechanics that make them tough to manfight. They also come with a veritable army of ''over 150 enemy units'' that include a good number of DemonicSpiders in their ranks. Unlike most encounters in Arknights, these units don't come in trickle allowing you to kill them piecemeal, but [[ZergRush will come in full force]] from the start to overwhelm your defense line. And unlike Integrated Strategies or SSS mode, the buffs obtainable in RA are a lot more modest in comparison, so you're in for a tough fight no matter what. It says something that you are ''not'' expected to defeat this army in one go, but rather to raid and whittle their number down as they make their way to your HQ for the final confrontation, and hope you've weakened them enough to finish them off.

* Mandragora from Chapter 9 can be a tremendous pain in the rear mostly due to her Stoneshield, which drastically reduces Physical and Arts damage taken, and can only be broken by dropping a Tattered Pillar on her. The Pillars in question are single-use and not always placed in convenient locations, making it very possible to permanently waste your chances to bring her down - not helping is that the levels she's fought in will often demand some of the pillars be used for rerouting or killing elite enemies, further limiting your chances at breaking the Stoneshield. Once you crack the shield, you have only 25 seconds to lay down the hurt on Mandragora before she grows it back, and she'll also instantly regain it if she takes enough damage at once, just to make bursting her harder. Offensively, she's also a hard hitter with alarmingly fast Arts attacks and Mandragora's Gaze, which deals massive continous damage while crippling the target's ASPD and potentially sealing the tile if the target is retreated under its effect. However, once you take out her first health bar, her second phase is the real kicker, as she'll start to ''fly'' while gaining a FlechetteStorm aura, ranged attacks, and the ability to manually bring down pillars on top of operators for a OneHitKill. The flight can be extremely damning as it'll make her unblockable and immune to non-ranged attacks, rendering her near impossible to stall or stop unless you can bring her down - and bringing her down is now harder since she can fight back at range and use the Pillars against you, wasting your chances of defeating her while disrupting your formation and making it even harder to fight back.[[note]]However, it's worth noting that her new ability to collapse Tattered Pillars can be an advantage, as she can be tricked into [[HoistByHisOwnPetard collapsing a Pillar onto herself. Additionally, Amiya's S3 is able to hit flying enemies and deal True damage at the same time, which is very helpful against the second phase where Mandragora has Low-altitude hovering and does significant damage enough to dent most of the health bar, if not outright defeating her.]][[/note]]