Items and Capabilities Required
* One or more Grand Soul Gems (empty soul gems are best, but some filled gems can be used, see notes)
* A method of casting the Soul Trap effect
The Procedure
1. First, head to one of these locations: Dark Fissure (east from the shrine of Vaermina), Fort Istirus (east of Kvatch), Fort Linchal (northeast of Kvatch), or Wendelbek (at the headwaters of the Panther River). Outside each of these locations, you will find a white altar draped in a red cloth
2. Wait for the correct day, namely the Shade of the Revenant (see below for details).
* If there is no light there when you arrive, you can use the wait command to wait 24 hours at a time. Or you can calculate when the next Shade of the Revenant will occur, go complete other quests, then return on the correct date.
* When waiting for the purple light, do not stand directly in front of the altar; back up so you can see the altar and a fair portion of sky. The purple light is very soft by the time it gets to the altar, and if you are standing directly in front of it you may not realize it is shining and wait right past it. Also, doing this at night will help.
3. Once the light is shining on the altar, place one or more grand soul gems in the altar (it's a container like a chest).
4. Cast a Soul Trap spell on the altar. When you open the altar again, one grand soul gem is now an empty black one.
5. Repeat step 4 for as many spare grand soul gems as you have. Only one will be transformed for each Soul Trap spell, but you can cast one spell after another.