With my arcane blade the latter happened when opening the first coffing, causing the necromancer to wake up.
I met her in the middle of the level... and decided to fight! (My arcane blade had pretty good equipment and there was a challenge!)
I got the necromancer and a few of the elites down before an another unfairness got me: one of the elites had the impending doom talent

The room of the necromancer should be like a vault, so that the random elites can't cause fatal consequences with teleportation talents.