Damien is a boss that can be fought inside Damien's Lair as part of the Good Main Quest
Strategy[]
Damien is stationary, so heroes can retreat to the back of his lair when they need a break, and attack with burst damage when mana and cooldowns allow.
When Damien says that the generals you faced are nothing compared to him, retreat because a huge explosion will follow that deals massive damage.
When Damien says that the dead will rise, a series of explosions of blood will emanate from the top of the screen and head down, so stay as high as possible or leave the lair if you need to dodge them.
Damien will occasionally summons Infected zombies.