Blackstaff (item)

The tern Blackstaff refers to both a clandestine position within the White Council, authorized to break the rules of magic, and a physical item.

The Blackstaff
In the course of Changes, Harry witnesses the current Council Blackstaff, Ebenezar McCoy, summon, from seeminly nowhere, a featureless, black length of wood. The Blackstaff is literally a black staff, similar in most ways to a normal wizard's implement, only where a normal focus would be carved with runes to focus the powers channeled through it, the Blackstaff is devoid of any markings.

Harry claims to have a distinct feeling that the Blackstaff appears to have a kind of consciousness all its own, seeming to know its purpose (primarily, killing) and to want nothing less than to carry it out as swiftly as possible. Harry also notes that the more McCoy uses the item, it appears to attempt to exert some kind of control over its wielder, as black tendrils seem to crawl up the arm of the user which appear to hurt the wielder and fade over time.

Conjecture
Numerous times throughout the series, Harry and others claim that magic can only be used to perform acts that the user believed in. Once Harry discovered his former mentor's actions as the Blackstaff, he was shocked by his capabilities to break the First Law of Magic despite his teaching Harry the philosophy of magic previously mentioned. It seems plausible that the Black Staff may be able to exert its own will in order to carry out actions that the wielder may not believe in.