To create one, take an empty Grand Soul Gem to an altar at Dark Fissure, Fort Istirius, Fort Linchal, or Wendelbek on the day of the Revenant Shade. This occurs every eight days starting on the first day of play. An astral beam of light will fall onto the altar at midnight. Once an empty Grand Soul Gem has been placed in the altar cast a Soul Trap spell on the altar; the soul gem inside will be transformed into a Black Soul Gem.