Aliens would be a fourth (fifth) factions, after Triffids, Slime Fungal and maybe AI. They come later in game, and are significantly more dangerous, being equipped with powerful ranged energy weapons.
Aliens came down to harvest energy from rifts in reality (portal). A typical alien base will be 2-3 ships (metal building, think X-Com’s saucers) as well as a few weird machine outside to harvest the energy. Aliens themselves would concentrate near the settlements, and come in a few varieties: workers (no real weapons), guards (good weapons) and overlord (good weapons, can call other aliens to itshelp). All of them wear powerful and though power suit.
In addition, the act of harvesting power tear reality itself apart, causing many netherworld creature to spawn.
Aliens provide good loot, but you’ll need to pack significant firepower and operate from the cover of darkness to get it, as a frontal assault would likely be suicide. Inside one of the ships, some kind of console-hocking/core-bashing should let you activate a self-destruct sequence to destroy the installation, causing the rift to mend and stopping more netherworld creature from spawning.