A Necromancer Altar is required to convert a Grand Soul Gem into a Black Soul Gem. One of these altars may only be used once a week on a certain night of the week. On the night when one may be used, it will be illuminated by a purple beam of light shooting downward from the sky (see Gallery). This beam of light is more noticeable further away from the altar; if the Hero is standing directly in front of the altar, it is easy to miss; however, if there is a Necromancer present, then it is most likely the right day (correct days end at midnight).