It is a (monster manual) for the Deathwatch RPG. In Deathwatch , players are Space Marines of the elite Ordo Xenos, dedicated entirely to hunting aliens. This book provides the game master with a arsenal of alien creatures, NPCs, and environmental hazards.
The Cadian Shock Troops would have to be cautious, for on Molech, the line between friend and foe had grown perilously thin. The Mark of the Xenos had been claimed, but at what cost? Warhammer 40K - Mark Of The Xenos.pdf