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.