The Ritualist Spirits spawn at random locations throughout the dungeon. They can be destroyed, but will respawn after a set period of time.
The Dungeon Key is located in the first large room to your right, which houses several groups of Stone Summit and Undead. Make sure to pull groups a reasonable distance away from the room as wandering monks will stop to heal injured foes, which may result in massive aggro.
There appears to be a hidden room by the area map. It can be viewed by standing close to the wall and zooming in all the way.
When approaching the room containing the end boss, two lone Shock Phantoms appear to be guarding the entrance. However, upon acquiring aggro on the phantoms, several more will pop up. Once these are dispatched, several more groups of undead creatures (including Crypt Slashers, Shock Phantoms, Wraiths, and Restless Dead) will spawn. Once these waves are defeated, the door to the final boss will open.
The final wave is several Restless Dead, which can take an unsuspecting party by surprise. They use armor-ignoring touch skills and shadow step to targets. Focusing fire on them will take them down quickly.
Selvetarm himself is not hard to kill, but he is accompanied by necromancer allies that will animate minion after minion. After he and his group are defeated, a few Restless Dead will spawn along with a dungeon chest. The chest will drop either a gold weapon, an Onyx Gemstone, or a Diamond.