The Quantum Archangel was a being of nigh-infinite power born by the fusion of Angeliqua Whitefriar with the energies of the Lux Aeterna, the quantum foam that makes up reality itself.
In the Dusk Nave of the Midnight Cathedral, in the dark side of Earth's Moon, the Master attempted to calibrate the enhanced TITAN Array by using Mel's old friend Angeliqua as recipient for the powers the Array could siphon out of reality. However, he did not foresee the event of the energies and Angeliqua merging instead of reacting catastrophically, and the result was a nigh-godly being capable of rewriting history and creating pocket Universes, the Quantum Archangel.
As her first act, the Archangel bestowed the Master with a War TARDIS with which to fight off the Divine Host of the Chronovores, who were hot in pursuit of him. Slowly, but surely, the Archangel set off to create a perfect cosmos free of suffering and despair, creating pocket Universes for each living being and ensuring each received the best life possible.
However, even beings as powerful as the Archangel required massive computational power to administer her abilities, and she chose to use the Mad Mind of Bophemeral as her personal computer. Even so, she failed to realize the Universe holds limited quantities of "reality", making her plan impossible, and that the Chronovores exist specifically to devour alternate timelines such as the pocket Universes the Archangel intended to create.
To avoid starvation after the Master's act had cut them off from their natural food source, the Lux Aeterna, the Chronovores started spreading their meals, degrading and corrupting each tiny Universe instead of outright devouring them completely. Victims of this included the Sixth Doctor, Melanie Bush, Arlene Cole and Stuart Hyde, each of which lost their perfect Universe. Each was saved by Kronos, a Chronovore, and The Doctor's TARDIS working in concert. Free to act against the Archangel, the Doctor was forced to bathe in the Lux Aeterna as well to reach the Archangel in any meaningful terms.
Distracted by the efforts of doing battle with the godly form of the Doctor, the Archangel didn't notice when Kronos attacked Bophemeral and shut it down, robbing her of the added computational power and giving the Doctor the needed chance to convince her to give up the power of the Archangel and send back the power of the Lux Aeterna where it belonged. Tearfully, the Archangel did so and returned to being Angeliqua. (PDA: The Quantum Archangel)