Asp Bow

Although ornate and intricately carved with serpent imagery, this bow seems utterly mundane. It reveals its true character when a wielder notches an arrow, at which point the arrowhead assumes the character and likeness of a hissing, venomous snake. Arrows launched from this bow are always made poisonous. However, there is a chance with each use that the arrow turns and bites its wielder on the hand. Vampires, and the Undead in general, are immune to such venom.

History
Tomb robbers unearthed the Asp Bow in the city of Khemri, along with several other artefacts bearing the marks of a Nehekharan snake Goddess named Asaph who claimed dominion over magic, beauty, and vengeance. When the tomb robbers returned to the Old World, a strange curse seemed to fall upon them, and all those who possessed items from the expedition died mysteriously. The Asp Bow eventually found its way into the hands of Neferata, and it is now used by her most favoured assassin.

Source

 * Warhammer Fantasy RPG 2nd ED -- Night's Dark Masters (pg. 122).