This ritual allows a character to create a black soul gem. This knowledge is rare, and only more experienced students of Necromancy, who have accessed certain profane texts, might know of it.
Creating a black soul gem requires waiting for the coordination of the stars to shine their light on a pre-prepared necromantic altar. This profane starlight is known as the Shade of the Revenant. Preparing the altar requires a Necromancy skill test with a +10 bonus, and takes roughly six hours. The necromancer must then place an empty grand soul gem onto said altar, and perform a five minute incantation, which requires another Necromancy +10 skill test. At the culmination of the incantation, the character must cast some form of Soul Trap magic on the soul gem itself.
The time at which the Shade of the Revenant occurs is unknown to most, but remains hidden in certain dark necromantic texts. Determining the appropriate timing for the Necromancer’s location requires a Lore skill test made at a -30 penalty, and should only be possible if the character would have been exposed to such texts. If everything is done correctly, the grand soul gem will be transformed into a powerful black soul gem.