The Doppelhander is a living sword that wanders in to a persons home and lurks quietly in a corner for some time, watching the person and learning their mannerisms. Once it has confidently learned enough about the targets routine, it appears before them and begins to transform into them, growing copies of their face and hands in a gruesome display. Before it finishes turning its blade in to a pair of legs, it kills the target - usually with a quick slash across the throat or a stab through the heart. Once it has killed its target, it takes over their life, with their friends and family only ever slightly suspicious something is wrong.
To prevent a Doppelhander from killing you and taking your place, you must be observant around the home. If a mysterious sword appears among your things, throw some salt on it to scare it away.
Mentioned in Hellswing's Cryptid Casefiles. See: hellswing