Black Soul Gem

From the OblivioWiki – your guide to everything Oblivion

Black Soul Gems are a special type of Soul Gem that can contain humanoid souls (Imperial, Argonian, Khajiit, etc. - including Dremora). All other soul gems, including Azura's Star, hold only the souls of creatures (animals, or the non-humanoid daedra found near or inside Gates of Oblivion). Black Soul Gems filled with a humanoid soul are just as powerful as Grand Soul Gems filled with a Grand soul. These items are considered evil, as they are evidence of wrongdoing in the quest Cheydinhal Recommendation. Since Hannibal Traven was appointed head of the Mages Guild the practice of Necromancy has been banned along with the use of Black Soul Gems.

Black Soul Gems are created by placing a Grand Soul Gem in one of four Necromancer Altars in Cyrodiil, and then casting Soul Trap on the altar. This must be done on a specific night when the shrine is active (it varies from shrine to shrine). (For more information on creating a Black Soul Gem refer to the article on Necromancer Altars.)

Black Soul Gems can also be found on random necromancers or in chests (although this is rare).