YMMV / Eternal Card Game

• Game-Breaker: Piercing Grief, at first, looks like a pretty bad unit: it's 3/1 for 2, and it has Charge... which is good, because it dies at the end of your turn. However, it also has Lifesteal... and Revenge, which makes it come Back from the Dead, for free, once. At that point, it starts looking more like a spell: "You gain 3 life. Your opponent takes 3 damage. Your opponent may redirect this damage to a creature." And you can play this eight times, while your opponent can't do anything but counterspell it or sacrifice creatures to block it. The card advantage alone is absurd, before we contemplate the 24 Health you gain from it. Add a Xenan Cultist or two to the mix and things get really hairy.
  • Madness is a pretty basic Shadow spell that lets you take control of an opponent's unit for one turn. However, Shadow is the color that does the Bad Boss thing of sacrificing units for long-term gain using things like like Devour, Combust or Brimstone Altar. Using such a combo on an important enemy unit can be enough to swing the entire match in your favor. And then, to add insult to injury, the sacrificed unit goes to your void, and can thus be resurrected by Back from the Dead spells like Sleeping Draught or Dark Return...
• Serial Numbers Filed Off: A number of Unit abilities are identical to those from Magic, just with different names: Charge is Haste (as it is in Hearthstone), Deadly is Deathtouch, Lifesteal is Lifelink, Overwhelm is Trample, Empower is Landfall, Exhaust is "tap" (but without the tilting, which is how the game avoids Wizards of the Coast's patent lawyers), Ambush is (functionally) Flash, Infiltrate is Renown, Tribute is Morbid, Spellcraft is Kicker, Amplify is Multikicker, and the Strangers are Slivers.
• That One Boss: Jekk, the Final Boss of the "Jekk's Bounty" campaign. He starts the match equipped with Jekk's Revolver, a 2/6 weapon. Combined with the units he's playing, which you're desperately blocking, and which he's afterwards shooting with his revolver, this basically guarantees that you won't have a board for a while. So now it's Turn 5 and you've finally destroyed his gun... only for him to play a "Silence and Stun Every Unit My Opponent Controls" spell, for free, once it breaks. Oh, and the weapon has Warcry, so he's guaranteed to have an absolute minimum of 4 Strength on the table, and probably a lot more. This boss is so annoying that people have published decks designed solely to beat him.
  • Inquisitor Makto becomes this by the third fight against him, by simple expedient of having twice your starting Health and being able to scale so quickly that most decks can't keep up. Adding insult to injury, he deals your Player Character a Plotline Death at the end of the fight, meaning that your reward for winning is losing.
• That One Puzzle: One of the Quickdraw puzzles has a solution that relies on the fact that the AI inexplicably refuses to block one of your units, even though blocking would keep it alive. If you don't know the AI won't block, it can seem impossible.