Sovereigns are the leaders of the alien forces and a powerful species altogether
The sovereigns are a low numbered race, which stands at the top of the command of their alliance. Ryan doesn't know how the sovereigns came to be, just that they stand at the top of alien ierarchy, even above the birthmothers, who give birth to most other aliens.
Surprisingly, they are thin creatures with tentacles and very big heads. This happens because they have a very evolved brain, all their organs are in the head because, according to Ryan, it makes sending stimuli much easier. Anatomically the sovereigns are not meant for our world, so there is either a low gravity atmosphere in their spaceships, or their tentacles used for movement can't hold their heads' weight, hence they cannot walk or even stand. The sovereigns have "programmed" all other alien species, therefore they possess a high skill in biology.
Sovereigns communicate with the other species through complex energy signals, untraceable and unreplicated by human technology. An exception to this are the zombies, who are unable to obey the sovereigns, yet they do not attack aliens as they do humans. On the other hand, Sovereigns are able to communicate with Judases, who also obey the sovereigns. However, a Judas obeys the Sovereigns partly due to his own will, and should that change, a Judas may become disloyal, at least partly. Should a sovereign die, the forces under its command fall into disarray and seek out another sovereign to serve under. Also, sovereigns work better together, and throughout the story, masses of alien forces relocate towards a specific point where most surviving sovereigns are, fighting any human groups along the way.
According to Ryan, the sovereign physiology is neither organic, nor mechanical or energical. It is more of a mix of the three, like zombies and most alien species. Their body is mostly organic-like, with many modifications over the course of their history to amplify their abilities. Their body runs on a plasma-like substance, which is most likely a liquid form of the plasma energy the aliens use on many occasions. Ryan says that they have unlocked the secrets to their biology, and as a result they have modified their bodies to last for thousands of years. Even when their bodies reach their limit, they clone them and have found a way to pass their brain function into the new body, essentially cheating death.