The Plague Sword drips with venomous slime. No armour saves are allowed against wounds inflicted by a Plague Sword. In addition, a wound from this weapon will kill mortal creatures outright on a D6 roll of 4 or more, regardless of how many wounds they have. On a roll of 1–3 the Plague Sword only causes 1 wound. Note that this only applies to mortal foes, not to other Daemons, vehicles, Talos, Avatars or Wraithguard.
Plague Sword
Source: Codex: Chaos Space Marines (1999), p. 29