Yet another concept that's just arisen from my random doodling at work. ^^;

The basic idea, is evil dark shards of metal made from equally evil dark magic. It literally bonds to a host by digging into their skin, forming a carapace of sorts. Though it often fits around limbs without fully encasing the victim. It tend feeds of the host, slowly gaining control of them until the host until their corrupted into its puppet. It remains on them until the host is injured and the metal moves to another host. Which is something it does eagerly, since fresh meat is always appealing to it. Otherwise its nigh impossible to remove without horribly damaging the host anyway, and the metal with easily realise what's being done and latch onto the next person.