Pretty typical necromancer type.  So essentially a mage/alchemist who experiments on the dead.  So we're probably gonna have a couple of corpses in there, maybe a few jars of random body parts.  Maybe a/some sigil(s) painted on the floor/walls.  metallic torch stands etc.  Due to most of the things within the room it's likely to have a very vile smell.  Possibly the idea that they hear some chanting in an ancient tongue which eventually gives away his position.

The necromancer himself would likely have a very dark but calm persona about him.  Possibly more surprised about how long it took for them to arrive than the fact that they'd found him.

Is this enough to work with, or do you want more? ^_^