
A pentagram.
A pentagram is a five-pointed star.[1] In My Immortal, it is used as a symbol of Stanism. It is also used as an alternative, goffic, way to say C-R-O-S-S.
Vampire Potter has one of these covering his lightning bolt scar. The Hogwarts "Advanced Biology" class apparently has an exercise where people turn bloody pentagrams into guitars.