Necromancer Altar

A Necromancer Altar is required to convert a Grand Soul Gem into a Black Soul Gem. They can only be used once a week on a certain night of the week. On the night one can be used, it will have a purple beam of light shooting upward into the sky (see screenshot). This beam of light is more noticible further away from the altar; if you are standing directly in front of the altar, it is easy to miss.

Using a Necromancer Altar
To use a Necromancer Altar, put the empty Grand Soul Gems you want to convert into the altar on a night when the altar is active. Close the altar and cast a Soul Trap spell on the altar to convert one of the Grand Soul Gems into a Black Soul Gem. This process is repeatable - cast Soul Trap as many times as you have Grand Soul Gems to convert. (See screenshots for the before and after)

Necromancer Altar Locations

 * Fort Istirius - active on Morndas
 * Fort Linchal - active on Sundas
 * Dark Fissure - active on Fredas
 * Wendelbek - active on Turdas

Azura's Star does not succesfully turn into a black soul gem.