This small brooch or pin of magical nature appears to be any one of the various amulets, brooches, or scarabs of beneficial sort.

However, if it is held for more than 1 round or placed within a container (bag, pack, etc.) within 1-ft of a warm living body for 1 turn, it will change into a horrible burrowing beetle-like creature. 

The thing will then tear through any leather or cloth, burrow into flesh, and reach the victim's heart in a single round, causing death. 

It then returns to its scarab form.

(Hard wood, ceramic, bone, ivory, or metal will prevent the monster from coming to life, if the scarab is secured within a container of such sub-stance.)