Black Soul Gems or Illegal souls gems are soul gems that can contain the souls of "higher" creatures, such as men or mer. They are associated with the art of necromancy, and the God of Worms, Mannimarco, who also has the power to transform white soul gems into black.
Black Soul Gems were developed either during or at some time close to the Interregnum, through a ritual known as the Shade of the Revenant, in which Great or Grand Soul Gems are offered to Mannimarco. Black Soul Gems can also be created using lightning attractors in the Soul Cairn, although the precise mechanism for the gem's transformation is unknown in this case.
Black soul gems can be used to trap the souls of sentient beings, which circumvents the law of Arkay. It appears that souls trapped using Black Soul Gems are held in Oblivion, in a realm known as the Soul Cairn. While the gem holding their soul is intact, those spirits cannot technically die.
Black soul gems also appear to be "hungry" in a way that white soul gems are not; they are noted to potentially absorb parts of a handler's soul if they do not contain a soul of precisely the right size within them.