The Avatar is an alien enemy encountered in XCOM 2. They are the ultimate goal of the Avatar Project, lead by the ADVENT occupation on Earth: a new series of bodies for the Elders that are far more healthy than their decaying Ethereal ones, but still exponentially more powerful psionically.
While very powerful, the Avatars encountered in game are incomplete; without the entirety of the Avatar Project, their bodies are not nearly as potent as a completed Avatar is presumed to be.