EE:Lavavein
From DDwiki
Lavavein, along with his partner, powers up every kill the player makes. He gains 1 damage per level of the last monster slain, which puts a pressure to take him out early. His rather high health makes this difficult, however, without significant bonus experience to help, which makes 
IMAWAL an amazing find, and the Goblin race a worthwhile pick.
The most dangerous part of Lavavein is how First Strike bolsters the new Unstable trait. Unstable will hit you for as hard as the monster had remaining health on the death blow, which is stacked on top of Lavavein's normal attack. Without a way to slow him, or clever manipulation of his health, this can make him nearly impossible to finish off.
