Necrons would make the most sense, with their lower ranks being very literal NPCs. Alternatively, we could have Malal cultists, because Malal hates everything.
Imagine the collective rage as Ultramary Sueonius is required to gather twelve boar feet. And all boars are mutated by Chaos, having tentacles instead of feet.
so evil and the mutated boarwolfking will take over our bases, steal our ships and leading his own crusade to terra!
My vote is conflicted with the Necrons and the 'Nids. They both seem like good NPC races, but the Nids would make more sense since they have a swarm-like mechanic to it while the necrons slodge around and *Necron Beeping noises*
Necrons.... But I'd love to see some force from the lore who's yet to be fleshed out correctly. It'd be an unpopular move but slaugth would be so cool to stumble across.
Although Malal isn't GW licensed due to the Creators being dissociated with GW, so Malal isn't happening