Brutal Green-skinned creatures whose only goal appears to be to annihilate anybody who are not them... Because its funny!? They are nearly numberless, and they have taken a great toll on us.
Tyranids
Tyranids are bug-like creatures who's only goal is to devour every bioform known. They are countless numbers of them, and they can take you down easily in large numbers.
Chaos Space Marines
Corrupted Space Marines who joined the Dark Gods and crusade against everybody who is not them. There are not many of them, but they are incredibly powerful.
