I would have to pick Archimonde's Hand of Death spell from World of Warcraft. What happens there is after 10 minutes, if Archimonde has more than 10% of his 3.2 million health pool (used to be more, but was lowered near the release of Wrath of the Lich King) remaining, he his what's known as a berserk timer where in any normal situation the enemy just does a lot more damage and hits a lot faster and inevitably kills everyone in the group. When Archimonde hits his berserk, he just instant-kills you. It is literally a one-shot kill, dealing 99,999 shadow damage, completely incapable of being mitigated by spell damage resistance or armor.
Alternatively, he has another spell called "Finger of Death [http://www.wowhead.com/?spell=31984]" that does 20,000 damage to one character when he has no targets in melee range. Back when Archimonde was first introduced as a fightable boss, the only characters that would have over 20,000 health were the people responsible for keeping the boss's attention while damage-dealers picked away at his health (AKA the tanks) and even then, they only had that much health when their stamina was amplified out the wazoo. Essentially its real name was "Middle Finger of Death." Even now, when the level cap has been raised to 80, Archimonde is a very difficult raid boss because of a constantly-spreading fire spell he casts every X amount of seconds and a fear ability that will inevitably send you running into said fire that will do almost 10,000 damage and most likely kill anyone it touches.
Alternatively, he has another spell called "Finger of Death [http://www.wowhead.com/?spell=31984]" that does 20,000 damage to one character when he has no targets in melee range. Back when Archimonde was first introduced as a fightable boss, the only characters that would have over 20,000 health were the people responsible for keeping the boss's attention while damage-dealers picked away at his health (AKA the tanks) and even then, they only had that much health when their stamina was amplified out the wazoo. Essentially its real name was "Middle Finger of Death." Even now, when the level cap has been raised to 80, Archimonde is a very difficult raid boss because of a constantly-spreading fire spell he casts every X amount of seconds and a fear ability that will inevitably send you running into said fire that will do almost 10,000 damage and most likely kill anyone it touches.
