Dark Fissure (Oblivion)

Dark Fissure is a cave located southeast of Cheydinhal, and east of The Reed River. Inside the cave you will find various undead creatures (zombies, skeletons, lichs, etc.) and human necromancers, as well as your standard dungeon loot. For those of you interested in Black Soul Gems, it should be noted that the cave doesn't have any Black Soul Gems to loot (as was my experience after searching this cave in its entirety) although there may be a small chance to loot one from a necromancer.

Outside of the cave is a Necromancer Altar which is used to create Black Soul Gems. Once a week a light beams down on the altar, but which day of the week this happens varies from game to game. (For starters, try it on Fredas as this has been the active day for other players.)