https://www.pcgamer.com/meet-the-new-enemies-in-rainbow-six-siege-outbreak/

"It's adapting faster than we can track," Rainbow Six demolitions specialist Eliza "Ash" Cohen says in the Outbreak briefing trailer Ubisoft released last week. "We up the ordnance, and it created this thing." A thing that looks like it would be more at home in Prey than a Rainbow Six game, but there it is—and there are actually five of them, each with unique characteristics and abilities. 

The otherworldly bad guys are actually people, transformed by alien parasites into unearthly creatures who presumably exist only to kill/transform every non-alien-parasite thing they see. Zombies from space, basically, although whether or not this infection can be cured, and thus the morality of gunning down its victims before that question is resolved, is unclear.