The Theleans are, in short, a race of slug-like creatures who were uplifted by a Centens called Rashaek, much like how the Avikan were uplifted. Unlike with the Avikan though, Rashaek only uplifted a single Thelean: Creaton. Creaton then proceeded to uplift his race in Rashaek's name, and became the leader of the Theleans. He created mechanical endoskeletons for his people, changing them from slug-like creatures to more humanoid ones. The Thelean Dominion eventually grew to become the single largest nation in the galaxy, and their level of technology is nearly unsurpassed by other modern nations.

Creaton still rules the Theleans to this day, and his ultimate goal is to bring the Centens back from wherever they vanished to. He hoards Centensian relics and hunts down those who hold relics themselves.

There's a lot more to the story, but that's a very brief overview. If you're interested in learning more about the lore, I recommend you check out the Elithian Alliance Discord server, which has channels dedicated to lore discussions!