Warning: This is 3.0 Edition material. It is possible that there is more recent 3.5 Edition version.

Death Grimace

(Book of Vile Darkness)

Necromancy [Evil]
Level: Assassin 1, Blackguard 1, Sorcerer 1, Wizard 1,
Components: S,
Casting Time: 1 action
Range: Touch
Target: One corpse
Duration: Permanent
Saving Throw: None
Spell Resistance: No

The caster marks a corpse with a particular magical "signature", thus identifying himself (truthfully or falsely) as the murderer.
Some assassins use this signature as a sort of calling card, while others use it as a way to prove that they earned their pay.
The signature can take many forms: a specific expression on the face of the corpse (hence the name of the spell), an actual rune, a word written on the corpse's flesh, a discoloration of one eye, and so on.