Viper

Vipers are a sapient snake-like species resembling Gorgons of ancient Greek mythology. Vipers were first encountered by humanity in the form of 'Thin Men', a genetic modification resulting in the Vipers appearing as identical Human males in formal clothing. Vipers also produce an extremely potent venom which they are capable of expelling at long range.During the Ethereals' invasion of Earth in 2015, Vipers were extensively modified to serve as infiltrators, becoming vaguely Human-like creatures known as Thin Men. XCOM were able to identify some markers of the Vipers' true form in the thin-men, such as their reptilian eyes, ability to spit toxins at long range and their serpent-like bone structure.

After Earth was conquered, the Vipers made up part of the occupation force and engaged XCOM and resistance forces around the globe, demonstrating a disturbing tactic of capturing hostile Humans with their extremely extendable tongues and constricting them with their own bodies, suffocating them. Throughout XCOM's liberation campaign, the alien occupation seemed to only field female Vipers in their ranks, this is because the Ethereals did this to restrain the Vipers from reproducing, although at least one male was subject to Dr. Vahlen's experiments. This subject dubbed the "Viper King" was genetically modified to have quick reflexes, ice-filled venom glands and a considerable strength increase. The Viper King immediately started reproducing with the standard Vipers, giving birth to the "Neonate Vipers" in its nest, a facility previously used by Dr. Vahlan. Once it was killed, the Viper King's corpse was used to make an experimental armor called the "Serpent Suit".

After Earth was reclaimed and the ADVENT coalition was collapsed, the remaining aliens left stranded on Earth, including the Vipers, were allowed to live in relative peace with Humans. One Viper, Torque, was enlisted as a member of XCOM's Chimera Squad to help maintain the peace. Others found less lawful employment as smugglers in the Vipers' Nest, scavengers of Grey Phoenix and mercenaries of Shrike.