I Choose You, Runemaster Molgeim (25 player)

Obviously a reference to "Pokemon". Whenever a trainer would send out a pokemon, they would say "I choose you <name>!"

I found it pretty funny. :)

Killing a boss of the Iron Council Supercharges the remaining bosses, healing them to full, increasing their damage and giving them new abilities. To get this achievement, Runemaster Molgeim must be killed last, which is considered the first tier of hard mode.

Molgeim starts off with the Runic Barrier and Rune of Power abilities.
When 1 other boss is killed, he can cast Rune of Death, a void zone.
When both other bosses are killed, he can create a Rune of Summoning, which in turn summons suicidal Lightning Elementals, exploding with Lightning Blast.

If he or Steelbreaker is killed last of the 3 them drop a Archivum Data Disc.
This item starts the quest for the Algalon Key.

To get the key you need to kill Thorim, Freya, Hodir and Mimiron on hard mode.