Medivh is a reanimated dead mage sealed away in the Mage College, complete with minions, spells, and evil laughter.
Medivh starts with an excruciatingly long monologue before battle. Afterwards, he attempts to kill you all with an endless barrage of magic beams, minions, and fireballs. They will also levitate you from time to time, making it harder to reach him. Medivh has no separate healing source, but instead heals himself at intervals, so you don't have to worry about dividing the team. At his second stage, he powers up and continues to summon more minions and shoot more beams. Be sure to keep an eye on his health, as it could heal back up fully while still in the second stage.