Cryptosi

The Cryptosi were an exotic meta-dimensional Xeno species that came into conflict with the Imperium, after attacking several of its colony worlds, during The Great Crusade. What became of the Cryptosi after a Sub-Company of the Alpha Legion was sent to investigate the atrocities they committed is unknown; though the Imperium soon lost contact with the colony worlds they attacked, due to the actions of the Legiones Astartes.^[1]

Source

• 1: The Horus Heresy Book Three - Extermination, pg. 94