The final category of factions are defined by their lack of humanity, the Xenos. What the Imperium defines as alien, the Xenos are not one race but a series of unique factions that have vastly different physical appearances and motives.
The Tyranids, also known as The Great Devourer are insectoid creatures that travel the galaxy in hive fleets to infiltrate a planet with Genestealer Cults before arriving to consume.
The Necrons are a race of mechanical warriors, who were once Necrontyr until Mephatโran the Deceiver tricked the Silent King into the biotransference of their kind into Necrodermis bodies. Now in their bodies of immortal metal, the Necrons utilise their reanimation protocols to send endless hordes to fight for them.
The Orks are an unorganised and rowdy faction who, whilst less advanced than their other Xenos counterparts, persist through brute strength and the power of WAAAGH! Groups of Orks are led by the biggest of their kind and can will things into existence due to psychic powers they are unaware of.