I did not assign a rarity to the staff because only a necrarch may bear one. It is assumed that these masters of ancient magics craft them themselves, and the price is merely there to represent the cost of doing so. Upgraded Thralls may craft one for themselves should they find themselves in the captains seat (so to speak).
If you'd like to add them to random treasures found for whatever reason, a rarity of 8 seems low for it's capabilities (and scarcity). I'd go with 10+ (but of course feel free to house-rule it as you see fit).
Sorry for the delay in response... at this rate I'll be caught up by 2267!