It looked like thin metal "wand," but once an individual was stabbed with it, their biodata would be purged from the Spiral Politic entirely, and they would never have existed. It required a secret code to operate. The Order's chatelaine, Thessalia, considered the needle one of the Imperator's many mistakes. After the chatelaine's death, the needle became Larissa's possession, and she planned to use it to destroy the babel.
When Larissa stabbed Doctor Bendo with the needle, the Spiral Politic had to rewrite itself to compensate for his absence, like "patching a hole in a bucket to stop it leaking." When the needle nearly erased Jeova Unus Sanctus from history, time actually began to unravel.