From Exterminatus Manual
It is the 41st millennium and Mankind has reached the stars, however dangers untold and unimaginable inhabit the depths of space, ancient powerful races bent on their own survival, machines from the beginning of time and even Gods stalk the darkness between the stars.
This is not a safe galaxy to live in; the Imperium of man is large and beset at all sides by forces wishing humanity's destruction. Standing against this are the armies of mankind led by the Imperial Guard made of men and women, faith and steel, fighting the endless battles that must be fought.
Sometimes even this is not enough and in those times the Adeptus Astartes, The Space Marines, are called upon to defend humanity. Genetically modified warriors, equipped and trained to the highest standards of the Imperium to fight out-numbered and out-gunned and win. They are the Elite of the Elite.
All the while the God Emperor of mankind sits trapped in un-death atop his golden throne at the heart of the Imperium -- known to antiquity as Earth -- Terra.
This is a savage age. Forget being one with the cosmos, forget all you think you know about aliens, this is the Grim Darkness of the Far Future and there is Only War.
A vast intersteller intelligence threatens our galaxy, the Tyranid hive mind. The swarms of creatures that make up the tyranid hive fleets have attacked before, only to be halted at great expense. Never before has unamity been threated by a fleet as large as Behemoth. The insaitable hunger of the hive fleet consumes all life of any world they encounter, the only way to quench this fuel is by exterminating all life on these invaded worlds...
Exterminatus is inspired by Games Workshop's(GW) table top war game "Warhammer 40,000", sometimes known as 40K. The Exterminatus team recognizes all GW trademarks and copyrighted materials and thanks them for letting us share in their wonderfully involving vision of the distant future.
Exterminatus VS 40K
Exterminatus is set in the Warhammer 40,000 background, but is not tied to any particular edition, rulebook or codex. Occasionally the need to make a fun FPS conflicts with some aspect of the background, or particular edition of the rules. The Exterminatus team believe this is inevitable and have attempted to document their reasoning for these diversions from pure 40k koan in article sections titled Exterminatus VS 40k.