R'tas is one of the Celestials, the Celestial of Order. He was created as a response to the dark chaotic energy of Legion, and thus is his polar opposite. He is the Conservator of the Convergence Multiverse.
R'tas came into being during the great war against the Celestial of Chaos, Legion. His role was to use his unique powers to counteract the effects of Legion's corruptive essence on the fabric of reality.
He was largely successful in doing so, winning most engagements and encounters he and his army of Eternals had against Legion and his forces. His victories truly turned the state of war from being a stalemate, and the scales were tipped against Legion.
Eventually, combined with the collective might of his Celestial brethren, trillions of Eternals and entire armies of technologically sophisticated mortals that were unfathomable in number, R'tas and the other Celestials were successful in defeating Legion.
R'tas was the one to use his powers to create the pocket dimension in which Legion had been imprisoned, and he was also responsible for banishing it from the universe. Despite being forgotten about, by the rest of the universe, R'tas keeps an eye on the prison constantly, while he goes about his task of being the Conservator of the Convergence Multiverse.
In the billions of years following the war, R'tas rarely ever interacted with any of the universes. Often times however, he materialised himself in front of mortals to send them on quests through time due to the level of importance of these missions. Although mostly, R'tas keeps to the realm between the Multiverses, and is quite distant from even other Celestials.