Given that the mod is generally seen as a huge Difficulty Spike compared to the vanilla game and even many other mods, there are many of these to be found at all stages of the game.
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Calamity Bosses
- The Slime God. He's extremely difficult for the point of the game that you fight him at and is a large step up in difficulty from any of the bosses fought at the point. The Ebonian/Crimulan Slime Gods are prime examples of Lightning Bruisers: they have decent bulk, powerful physical and ranged attacks that inflict debuffs (Darkness for the Corruption slimes, Weak for the Crimson slimes), very high mobility, armies of minions that spawn when they're damaged, and the ability to split into 2 at half health. The large slimes will jump all over the place, soaking up hits while dealing ridiculous damage in return using their contact damage, minions, and fast shotgun sprays of slime balls. The invincible Core itself isn't as much trouble, but it's small and sometimes hard to see in the chaos, not to mention that it can and will take potshots at you to inflict you with both of the above debuffs and Slow simultaneously, which can really ruin your day if one of the lesser Slime Gods are nearby. In addition, the Core can occasionally possess a larger God, jacking up its stats for a short time and letting it run you down. And depending on which God you kill first, the Core will become even more aggressive while the remaining one gets unique buffs, with the Crimulan and Ebomion Slime Gods doubling their attack rate or defense respectively. Noteworthy is that the Slime God was voted fourth place in a community poll about who was the hardest boss despite being a pre-hardmode boss.
- Anahita and the Leviathan, albeit being more like a Superboss, are an extremely difficult Dual Boss fight against the Glass Cannon Anahita who is deceptively fast and hits very hard with both physical and ranged attacks that inflict Status Effects (notably projectiles that spawn around you instead of being shot directly), and the Mighty Glacier Leviathan (which joins the fight once Siren is at 40% health) who is slow but incredibly bulky, takes up most of the screen, and deals ridiculous damage with both rams and boulder shots that shred defense, all while summoning minions. While they're normally fought one at a time, both bosses will become active simultaneously once the Leviathan is brought below 40% HP as well. Managing both bosses and the constant swarm of minions can prove to be extremely difficult, especially since Anahita and the Leviathan cover each other's weaknesses extremely well. Oh, and whichever one is the last one alive will become enraged. No wonder they were voted the fifth hardest bosses in the entire game.
- The Calamitas Clone isn't too bad at first, although she can deal enormous damage and inflict the deadly Brimstone Flames debuff if you aren't careful. However, once you whittle her down enough, she'll summon a devastating Hold the Line Bullet Hell that is guaranteed to take you down a few pegs if you aren't prepared, before adding in lightning-fast charges to her previous attacks. The real problem arises when she summons her brothers later on; Catastrophe and Cataclysm are extremely fast, do tons of damage, and use flamethrower/bouncing fire attacks that take up huge portions of the screen. Even if you figure out how to dispatch the brothers, you'll have another bullet-hell segment to deal with before Calamitas finally lets you kill her.
- The Plaguebringer Goliath, a Lightning Bruiser with very high HP, study DR, incredible amounts of mobility (sometimes charging diagonally, something that can trip up players expecting a Queen Bee clone), and a combination of deadly contact damage and powerful projectile spam. Her attacks all inflict the Plague debuff, which can be annoying at best and lethal at worst. She also punctuates her own attacks with waves of deadly minions, which include homing missiles and Plague Mines that deal huge area damage on contact. The second part of the fight on Expert+ is even worse, as she now rains down missiles as she charges - not to mention that she'll gradually get stronger as you wear down her health bar.
- After receiving a massive rework, Ravager has become much more difficult. It has a colossal amount of health, deals stupidly high damage, and has a wide variety of attacks to hammer you with, including fireballs, homing flares, heat-seeking missiles, giant rock pillars, flaming lanterns that shoot fireballs at you, and of course its crushing physical attacks. All of these attacks will usually stack together to make patterns almost impossible to dodge. During its second phase, not only is it capable of terrifyingly fast, predictive jumps to slam down on players, it pulls out a few new tricks - not only is its head also chasing you around and vomiting even more projectiles than usual, you permanently have your flight time decreased by 50%note , forcing you to stay (mostly) grounded and face the brunt of Ravager's attacks.
- The Dragonfolly has a decent amount of HP, is blisteringly fast, and deals enough damage to two or three-shot an appropriately geared player on top of having no real pattern to its erratic attacks. Not only can it spawn clones of itself that are just as fast and only hit slightly less hard, it can also summon waves of homing feathers which can either chunk you for a good amount of HP or leave you open to an even more damaging physical blow - to add insult to injury, the feathers inflict Electrified in a fight where you absolutely cannot stop moving. Its final phases are even worse, as although it loses its minion spawning abilities, it'll become even faster while also summoning lightning pillars to restrict your movement.
- Providence provides a first impression of what the endgame is like in Calamity, and serves as a harsh gatekeeper for many a player trying to get into the post-Moon Lord segment of the game. She is extremely bulky, deceptively fast, and shaves off enormous amounts of health with each attack, using an incredibly wide and varied arsenal of projectiles to harass you with. Her attacks are all tricky to avoid, inflict a potent Damage Over Time debuff, and tend to flood the screen with projectiles (particularly those where she goes stationary and launches a bullet-hell attack). As you lower her HP, she'll spam these attacks gradually faster while adding more to her arsenal, like an invincible crystal that rains harmful shards, a devastating Wave-Motion Gun that would put Moon Lord to shame, or even summoning weaker versions of the Profaned Guardians, which buff her stats and heal her until you kill them. And the icing on the cake is that she can inflict a debuff that greatly weakens your flight, in a fight where mobility is likely your best chance at survival.
- If you fight her at night, you'll get a chance to battle her in an even stronger state that dials all her stats up to an insane degree and empowers all her attacks, in exchange for dropping exclusive items.
- Polterghast. While its HP is on the low side, it deals extreme damage very quickly, slams players with a variety of highly lethal debuffs, and liberally spams beams that are rather hard to avoid. Worse still, while the instinctual response to a Plantera-esque boss would be to create a big arena to maneuver around it, Polterghast will actually get substantial attack and speed boosts the less tiles that are around it, meaning that making your arena too big may accidentally turn the boss into a full-on Lightning Bruiser - especially since it periodically use charge attacks that increases in intensity as it loses HP. Bursting it down is nigh-impossible since it has passive damage resistance that increases as it loses health. Since you have to fight it in the Dungeon and it can sometimes summon itself, you will be limited to a tiny area unless you set up an arena beforehand, and even then one has to be careful since the Polterghast Turns Red depending on how open the space is. Taken to an absolute head in its final form as the Necroplasm - not only is it over twice as fast as before and able to deliver ridiculously strong melee attacks, now you have to contend with 3 deadly mine-shooting Phantoms and a clone of the Polterghast that will launch coordinated charges with the boss itself.
- Much like the original Duke Fishron, the Old Duke is a real hair-puller. He takes the Duke's Lightning Bruiser aspects and turns them up to eleven - packing high HP, very sturdy damage reduction, lightning-fast speed and insane damage, on top of flooding the screen with projectiles/hazards and inflicting all manner of Status Effects on hit. To make things worse, his charge attacks strongly predict player movement, so if you don't make more evasive maneuvers to throw him off he will hit you. Fortunately, he'll stop to rest every 10 attacks and temporarily drop his defenses to zero, but more often than not the opportunity to attack will be squandered just trying to recover from his prior beatings.
- The Devourer of Gods is downright infamous for being difficult. He's massive, has obscenely high health, moves incredibly fast, and deals insane damage with just about every attack on top of inflicting God Slayer Inferno, especially upon contact: enough that getting hit by its head in will more often than not result in instant death. Standard worm tactics with piercing weapons? Does next to nothing since anything that's not its head or tail takes next to no damage. Thought you could just build a platform in the sky to deal with it like you did with Destroyer? Well, too bad it doesn't need solid blocks to move. Try and outrun it? Not only does it fire long-ranged laser bursts which occasionally take on unconventional patterns, it will always be faster than you. Mobility? Neutered since just being near it warps gravity and flight. Still not enough for you? Well, when you beat him down a bit, he Turns Red and enters his second phase, which boasts a more unpredictable AI, rapid-fire Godslayer fireballs, a teleporting charge attack, and the ability to turn invisible every now and then to bombard you with laser grids.
- Jungle Dragon, Yharon packs enormous health, shaves off nearly half your health bar with each attack, moves incredibly quickly for his bulk, and punctuates his attacks with torrents of flares with varying movement patterns, Bullet Hell style barrages of fireballs (some of which he can immediately combo into a charge while you're still dodging), and flaming tornadoes to cut off your movement. He also limits your space to a small area cut off by flaming pillars, making excessive movement risky even discounting all the projectiles and tornadoes. This is compounded by the fact that he has so many distinct phases that it borders on Confusion Fu - if you don't account for each phase switching up his attacks, you'll almost certainly die due to how hard he punishes mistakes. And once he reaches his second full phase, he completely swaps out his previous attack pattern and fires even more wide-area projectile patterns in tandem with physical blows, even using Teleport Spam near the tail end of the fight just to trip you up.
- Supreme Witch, Calamitas, whose difficulty is downright notorious in the modded Terraria scene. The moment her fight begins, she'll build her own arena, go invincible, and unleash an omnidirectional, screen-spanning bombardment of hundreds of fireballs that deal stupendous amounts of damage, sometimes killing players before she even starts combat. Then, you'll have to handle Sepulcher, a large projectile-spamming worm that can only be killed by destroying the 10 deceptively durable Brimstone Hearts in the arena corners - preferably before Supreme Calamitas herself gets a chance to fire off too many attacks. Then, when you actually fight her, it only gets worse: she's decently tanky, blisteringly fast, deals enough damage to swiftly kill even defensive builds, and spams all manner of projectiles like fireballs and explosive homing blasts which inflict lethal debuffs. Worse still, she'll pull off more tricks as the fight goes on, namely repeats of her first Bullet Hell survival phase with more projectiles gradually added in, summoning Supreme versions of her brothers that can flood the screen with shots, creating massive Brimstone Monster Orbs that chase players for the rest of the fight, creating dense rings of Soul Seekers to block attacks and rain hell, and even summoning the Sepulcher again near the end of her fight. All of these combine to make a brutally difficult, drawn-out fight where it's painfully easy to make mistakes, which is a bad thing when each one costs so much.
- Supreme Calamitas' alternate equivalent, the Exo Mechs, are just as bad if not worse. They're a Wolfpack Boss fight against the four titular Mechs that are extremely threatening individually, but will eventually gang up on you and combine their strengths to create an absurdly difficult attacking formation. Artemis and Apollo are two very fast fliers who constantly use evasive maneuvers to dodge incoming attacks while using devastating projectiles and charges, Thanatos is an enormous worm who obstructs your movement and unleashes Beam Spam while also only being vulnerable while shooting, and Ares wields four independent weapons that deal massive damage in wide areas, particularly its Wave-Motion Gun and Fantastic Nuke that can instantly end you if you're not careful. They're manageable at first due to their heavily deterministic attacks, but once all of them start fighting together and pinning you from multiple directions, managing all of them simultaneously becomes a chaotic nightmare where failing to keep track of all of the attacks will incur massive damage - and there are a lot of attacks to keep track of. Worse still, most of their attacks lead your movement, so just running away will often get you killed. Although doing enough damage eventually causes the damaged Mech to fight you one on one, they'll enter a second phase when alone which enhances their attacks and gives them new tricks: Thanatos spams even more lasers and starts using a Wave-Motion Gun that must be thrown off by staying near its head and giving it a chance to coil around you, Artemis and Apollo start to desync their enhanced attacks while using a massive laser and explosive charge combos respectively, and Ares fires faster and can trap the player in a rotating cage of devastating rays while raining fire. And just to add insult to injury, the last mech alive will go absolutely berserk, so you best hope you softened it up beforehand.
Vanilla Bosses on higher difficulties
- Revengeance Eye of Cthulhu hits way harder than usual, and it will enter its second phase at far higher HP to make sure it spends more time in its berserk charging phase, which is now faster and more sporadic. In addition, below a certain amount of HP it will begin spawning more minions and using fast horizontal charges, which can easily catch unprepared players off guard.
- Eater of Worlds in Revengeance is a fight that can easily take most players off guard. Piercing weapons normally make short work of the Eater, but in Revengeance its segments massively resist piercing weaponry. To make matters worse, it deals far more damage so using fast melee weapons is out of the question. And the icing on the cake is that its head segments now launch cursed flames, which can easily chunk your HP at this point in the game, especially in Death where the debuff now deals doubled damage.
- Skeletron was already a piece of work in the original game, but Revengeance Mode makes it even worse. Skeletron's stats are pumped through the roof and it can now use several debuffs - his hands in particular can nail you with Marked to turn your damage reduction into butter, which is a problem when Death Mode makes him spawn with four of them. And when his hands are destroyed, he can now start teleporting every few seconds, firing off a dangerous shotgun blast of bolts every time he does this. Oh, and he respawns his hands at 1/3 HP.
- Wall of Flesh in Rev+, who takes the "come prepared or die" aspect of the original fight to its extreme. There isn't much to say except he deals a lot more damage with all of his attacks, especially with him firing deadlier eye lasers and switching them out for far stronger death lasers below half health, although they start slower to compensate. With the increased HP of both himself and his minions, he can easily turn the fight into a battle of attrition where he perforates you with lethal lasers while you try to whittle him down. Oh, and in case you wanted to put some distance between you and the wall, it can dash at you to close the gap.
- In all difficulties, Destroyer gains a gigantic resistance to piercing weapons, so it's physically impossible to tear it apart with piercing like you would in the original game (especially with the mod's nerfs to Holy Arrows and the Daedalus Stormbow, which were essentially its kryptonite in vanilla, and its massive DR when first spawning so you can't target its coiled segments anymore). In Revengeance, it resists piercing even more by virtue of despawning projectiles after they pierce enough times, and its probes no longer drop hearts. It's also packing a lot more firepower - its contact damage is boosted, and its laser barrages are now far more calculated instead of a fairly randomized spray. And while its fire rate goes down as it nears death, it gradually starts switching out its red lasers with more lethal ones - blue and green lasers that can inflict Electrified and Cursed Inferno, respectively. The icing on the cake is that its mobility increases as its HP gets lower as well, with the worm flying at half health and gaining charge attacks at low HP.
- Revengeance Twins takes the conventional way of defeating them and turns it upside down. Normally, the idea is to kill one twin (usually Spazmatism) before the other to stop them both from being in phase 2 at the same time. However, in Revengeance any twin in phase 2 will actually be Nigh-Invulnerable until the other is also in phase 2, forcing you to fight both of them together at their strongest (in addition, they transform earlier to ensure that they enter phase 2 with a large amount of HP). They also get some new attacks to play around with in phase 2 - Retinazer's lasers are more precise, inflict a burn debuff, and gain slight homing properties, while Spazmatism switches its Cursed Flames for a Shadowflame breath that deals much higher damage. And when you bring one twin below 15% HP, it enrages - Retinazer gains the ability to lunge while firing a storm of homing missiles, and Spazmatism gets much faster while belching homing shadowflame blasts. And Kill One, Others Get Stronger is in effect - solo Retinazer fires nearly solid streams of lasers while still liberally using his missile-spamming dash attack, while solo Spazmatism's fireballs gain homing properties.
- Duke Fishron was already hard enough, but Rev+ makes it far worse. His durability is pumped through the roof, bolstering his defense, almost doubling his HP, and giving him invincibility between phases which now start earlier - while his damage and speed are also jacked up to insane degrees. Not only does this prolong an already perilous fight, it means that mistakes now cost far more - especially since several of his moves now inflict Marked. His final phase is buffed up as well, since he can now also spawn Cthulhunadoes to cut off your movement and packs new charge patterns, including a nonstop dash chain when low in Death Mode.
