Go to Dark Fissure during the main quest, there is a black soul gem in the altar outside (kill the Worm Initiate). The Worm Initiate should have instructions on him as to how you make a black soul gem.
[center]For Makaveli[center]
The Don Kiluminati
We Still Ridin...
GONE FROM SIGHT
BUT NOT FROM SOUND
My rank is no longer 187