It's a few alien races that call themselves the Collective. They have a sort of caste system, which is based on their either their skill in combat or tactics.
The weaker, more numerous species are the Graax, which serve as little more than manual laborers. Farmers, and the like.
There are three different fighting class species. I'll set up a full background for all of them later. For now, you just need to know:
Gratch are 8 foot humanoids with wings and keen intelligence.
Fenrir are wolflike creatures that stand about 5'6" on all fours and operate as lone killing machines, (will kill each other if not mates or potential mates and they're not very smart.)
Valkar are insect like and have chitinous armor.
There are two main intelligent leader species:
The Celestials, whose full grown form is the same size as the average 10 year old, but their least intelligent would easily outsmart our brightest genius.
The Sentinels, Who are slightly larger than humans and train their bodies and minds to be in peak combat performance.
The Celestials are usually the political leaders and the Sentinels have the tactical and combat brilliance.