Gatekeeper (XCOM)

Gatekeepers appear to be large masses of flesh adorned with tentacles, a form concealed underneath a shell of spherical armour which creates the illusion of them being robots. Gatekeepers are notorious for their major psionic powers which are believed to be on par with the Ethereals themselves. Their psionic abilities include the ability to drain the life force from others through physical contact and opening psionic gateways, both of which may reanimate the bodies of those killed in these ways.

History
Gatekeepers were fought by XCOM and resistance forces during the occupation of Earth under the Ethereals and ADVENT Coalition. They were presumably named based on the first Gatekeeper encountered guarding a psionic gate leading to the Ethereals' undersea fortress. Afterwards, they would be deployed as elite units of the alien occupation.

Five years after Earth is liberated from the Ethereals' control, a group of ADVENT remnants known as Sacred Coil acquired a psionic gate and sought to use it to reestablish contact with the Ethereals. However, upon activation, a Gatekeeper emerged and went on a violent rampage. XCOM's leading theory is that this particular Gatekeeper passed through a psionic gate at the same time XCOM eliminated the Ethereals, becoming trapped in transit after the psionic network collapsed and driven insane by the time Sacred Coil accidentally freed it.