Something like a symbiote from Spider-Man, minus the part where it has a mind of its own and turns the wearer against everyone else.
You can make any shape at all, be protected from most kinds of attacks, strike back with a dozen awesome weapons made from symbiote goo, and in some continuities the veteran symbiote-users have even developed the ability to drop tiny offspring onto others and control them. It is the dream weapon, worn not carried. Here when you need it gone when you don't, a large part of its dangerous allure.