The aliens are the primary antagonists of the game XCOM: Enemy Unknown. They invaded Earth under the command of the "Uber-Ethereal" in the year of 2015. Their technology renders Earth's most advanced technologies obsolete, until XCOM reverse-engineered most of their technology, attacking them with their own weapons. Not only did they use plasma weapons, they use other powerful weapons, such as hand-claws for the Berserker, cluster bombs and laser cannons for the Sectopod, even biological weapons like the Chryssalid and psionics like the Ethereals. The aliens were eventually defeated by XCOM and Earth was saved.
The aliens have a wide cast of alien species and robots among their ranks. From the lightly armed Sectoids to the giant Mutons. Each alien species are equipped with plasma weaponry, though some aliens also use futuristic grenades.
Battle vs. Demons (Doom) (by Monkey Doctor 33)
No doubt, the Demons are outmatched by the Aliens. Though the Demons are far more ruthless and intimidating, the Aliens' firepower is simply overwhelming. Individually, the Aliens' units are far less expendable and more specialized than those of the Demons' chaotic units. Even with the inclusion of the Demons' powerful Cyber Demon they are still utterly trumped by the sheer might of the near-invincible Sectopod. Not to mention that the Aliens are capable of working together despite their differences against their hellish opponents.