Birth Mother

The Birth mothers are a rare and very important alien species. The birthmothers are named as such because their only purpose is to give birth to the aliens species that make the alien invading army. There are more Birthmothers than Sovereigns, and every Ziggurat had at least one. They are giant creatures, and their only function is to take energy, and give birth to the servant species. The Birthmothers are the only fertile species of the alien forces, and they do not require mating to reproduce. Moreover, their offsprings look nothing like their mothers. This happens because the cells used for production are vastly different. This is due to the fact that Birthmothers were created by the Sovereigns. Their genetic material was edited numerous times over the centuries to create the efficient species they are now. It will be revealed at some point, that the Sovereigns come to earth every a few tens of thousands of years, and enslave any species they find an interest in, to copy certain attributes. Having done this procedure many times, they have perfected the Birthmothers' production process at this point. 
During the raid at the Ziggurat, the group runs into a birthmother, and kills it. The scientists however take samples ad parts of the alien to test, as well as pictures. Ryan also adds more information regarding them, as well as other groups that have info. 
While comparing infowith another group, it will be deduced that birth mothers are all the same, and only differ in their reproductive organs, which differ based on the species the Birthmother produces. 
During the final battle, Ryan (or another Judas) will reveal that the Sovereigns are desperate to wipe humanity out, and have forced the Birthmothers to produce aliens at a faster rates, at the cost of their health. While this is good for the future since it will endanger the aliens eventually, they now have a formidable foe against them, one that will become impossible to defeat should they leave them be. The destruction of the Birthmothers is the reason the last battle begins.