Take an empty Grand Soul Gem and a Soultrap spell to any of the four Necromancer lairs. There will be an altar which will only be active one day of the week. You may need to wait up to six game days. Place your Gem on the altar when it is active and cast Soultrap on it to make a Black Soul Gem. These are used for capturing the souls of NPCs. One is also required for one of the Daedric Quests as a tribute to the shrine.