You are the completion of the Ethereal's Avatar Project, a human-alien hybrid capable of remarkable things.
You gain the abilities of the XCOM series' various units in one convenient package. You have the strength of an Elite Muton, the shapeshifting ability of a Faceless, the technopathy of a Codex, the Psychic power of an Ethereal or Prototype Avatar, and so much more. You can use the abilities of human units, alien units, hybrid units, whatever, as long as it appears in the series. Through a combination of Faceless shapeshifting, the biotechnology used in the Seeker's construction, and your own psychic power, you are able to change your biology to whatever appearance or functionality you wish, and combine organic and cybernetic abilities together, allowing for things like psychic MECs with genetically engineered eyes.
You also gain the knowledge and capabilities to build the varied technology in the franchise, from sentient UFO navigational computers, to plasma weapons, to even the Avatar Project itself. You can grow the biological units from the franchise in vats, and build the mechanical ones yourself, giving you loyal servants to carry out your plans.
This power also treats XCOM-likes such as Long War, Xenonauts, and Terra Invicta as "part of the franchise" for the purposes of this power. Try not to think about the logic of this.