Mana Devourer

Official Overview (Blizzard)

When threatened, the Mana Devourer attacks its enemies with a variety of area effect attacks. Periodically during combat, the Mana Devourer will Coalesce Power,expelling all of its mana as Loose Unstable Mana. Players will need to collect the Loose Mana before it reaches the Mana Devourer. After collecting the Unstable Mana, the players need to clear its stacks by standing in Energy Void. If the Mana Devourer should ever reach full mana it will cast Decimating Essence, most likely killing the entire party.