So you got black sould gem http://www.gamesas.com/topic/1476719-necromancy-and-crime-system/ - paid a lot effort to create, but it is not end of story.

Now it is time to catch soul. Simply throwing soul catch spell does not work - it is how you achieve same as with grand soul and is just waste.

Two options:

- NPC

- other faction

In both you need three necromancers, and cage. You know these cages from Oblivion?

Next:

/NPC version/ you catch NPC into cage and need to transport into some place where real bad ritual /taking one hour/ must be done to trap soul. When seen by other NPC you got big crime. Also ritual can not be interrupted by other alliance.

/player version/ you catch other player and got 5 (10) minutes to trasnport into special ritual place. Player can logoff/login - does not matter, he will be trapped.

Cage can be easily damaged but by any other alliance that trapped (or NPC). Then worst thing happens - soul looting = choose any skills tree and take it's power into gem, so player looses it, but you have much more powerfull soul gem with power that relates to mastery of skill and will work similar to sigil stones - type of power depends on skill chosen.

The problem is that necromancy would be the source of most powerfull weapons. So it can not be so universal source of weapons. We could choose them staff creation mastery only.

By the way walking alone in Cyrodil during ghost hours would be realy risky.