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.
: If you're a player or a game master (often referred to as a Warhammer 40K player or a GM), verify if the content is official or third-party. Official content published by Games Workshop is considered 'legitimate' and can be used in games. Third-party content might be usable under certain conditions, but it's essential to ensure you're not infringing on any copyrights. Warhammer 40K - Mark Of The Xenos.pdf
So, arm your bolter. Sanction your psyker. Recite the Litany of Stealth. It is a (monster manual) for the Deathwatch RPG
The book moves beyond simple "shoot them until they die" tactics. It provides biological breakdowns for species, including: : If you're a player or a game
The bulk of the book is dedicated to the primary antagonists of the setting: the Tyranids, the Orks, the Eldar, the Dark Eldar, and the Tau.