Skrulls - The Skrulls see Earth as their rightful new home due in part to a prophecy among their people's extremist zealots. As such the Skrulls have been planting themselves among the population secretly to seize power from within. This plan has been complicated due to a similar ploy by the White Martians.
-Skrulls have the numbers and resources, but their abilities pale in comparison to the Martians.
-Skrulls are capable of advanced shapeshifting which includes transforming into both organic and inorganic objects along with selective transformation for either parts or the whole of their body.

White Martians - Hiding among humans, having experimented on them eons ago, creating the Metagene. A minority on Mars, the White Martians are angling to take Earth for their own and use humanity to overcome their own weaknesses.
-In terms of numbers, there is only approximately one Martian Terrorist for every hundred Skrull soldiers. But Martians are stronger, faster and overall make up for it in abilities. They however lack the resources and organization of the Skrull army and tend to be much more individually motivated.

The Reach - Chasing both Skrulls and Martians to add their unique abilities to their ranks.
-A dangerous hive-mind collective. The Reach assimilates their worthy foes with a Scarab turning them into one of their own. Those who are unworthy of this honour or put to slave labour or simply turned to nutrient paste for the hive.

Kroloteans - Overgrown space cockroaches, wandering goblin-like race.