Here's how I'd make the Blackstaff;
[-3] The Blackstaff
It is what it is: a weapon 2 staff
Indestructible
Hidden Agenda: see below
One-time discount: worked into the cost
Sponsored Magic: Dark Pact
The blackstaff is the physical manifestation of some nameless dark entity that gives more power to wizards
in exchange for using their power for destructive, lawbreaking spells. For some reason, the entity wants this
violation of the laws to continue to happen...
Standard sponsored magic benefits. Lawbreaking thaumaturgy at the speed of evocation.
The caster gets a +2 Lawbreaker bonus on all lawbreaking spells cast while using the staff; since he effectively
already has the maximum lawbreaker for spells he is trying to cast, he gets no lawbreakers. Unfortunately, his
personality (aspects) are still affected - though not nearly as corrupted as by normal lawbreaking magics. There
is only his usual personal responce to what he does, not any supernatural compulsion to change.