You all know probably 4 places in Cyrodill where grand soul gems could be converted to black soul gems.

There was actually no reason to do it, but may be there is some ancient knowledge that make better use of it - some black magic that only necromancers know and do better enchanted weapons.

But who likes the idea of trapping human souls, so necromancy was forbidden and IC squads are patrolling these places - better do not get caught or else get big bounty and NPC will not like you. Not mentioning that other alliance also might have a wish to do the same in special hours - will you fight them or join together in black ritual?