One was an archon. It was the Celestis most feared field agent and leader of the Investigators. (PROSE: The Book of the War) They sent it and Two to find out what had caused a Celesti to not only cease to exist, but to have never existed. Two was also assigned to watch One, as the Celestis believed it may have gone rogue.
In fact, One was secretly working with the Hermit to destroy Mictlan. The two of them feared the realm would attract the attention of the Swimmers and cause the universe to be destroyed. To that end they arranged for the Fendahl Predator to be released from its time loop prison. Since it fed on concept, and Mictlan was a conceptual construct, the Predator could not resist feeding on it. The inhabitant whose existence it first erased was the one that caused One and Two to be sent to investigate in the first place.