Lacrymole

The Larcrymole were a sentient Xenos species that were driven to near extinction by the the Emperor and the Legions during the Great Crusade. The Lacymole were vampiric shapeshifters, capable of changing their form at will after feeding on an individual. The Lacrymole's true form was that of a mass of flesh and limbs in constant motion with only its black eyes unchanging.

History
During the Horus Heresy, a Lacrymole was used at least once as an assassin by the forces of the Warmaster, this particular Lacrymole was able to assume the form of a Aduptus Custodes and infiltrate the planet Bastion where it was to assassinate the senators and nobles of the ruling class, an act that would have had the planet revolt against the Imperium and side with Horus.