For the Emperor, and the Long Vigil.
The Vigil is eternal. The Alien dies.
This article discusses the historical and mechanical content of the "Mark of the Xenos" supplement for the Fantasy Flight Games (FFG) version of the Deathwatch RPG. Games Workshop’s current 40K tabletop wargame rules (10th Edition) differ significantly, but the lore and tactical inspiration remain timeless. Warhammer 40K - Deathwatch - Mark Of The Xenos.pdf