The Quantum Archangel was a being of nigh-infinite power born from the fusion of Anjeliqua Whitefriar with the energies of the Lux Aeterna, the quantum foam that made up reality itself.
In the Dusk Nave of the Midnight Cathedral, on the dark side of Earth's Moon, The Master attempted to calibrate the new TITAN Array (based on the earlier TOMTIT project) by using Mel's old friend Anjeliqua as recipient for the powers the Array could siphon out of reality. However, he did not foresee the event of the energies and Anjeliqua merging instead of reacting catastrophically. The result was a nigh-godly being capable of rewriting history and creating pocket Universes, the Quantum Archangel. This was caused by Kronos, who had altered history to force the creation of the TITAN Array, but refused to condone the murder of an innocent and instead allowed the fusion to avert Anjeliqua's death.
As her first act, the Archangel bestowed on the Master a War TARDIS 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 lived 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. She failed to realise the Universe held limited quantities of "reality", rendering her plan impossible. In fact, the Chronovores existed specifically to devour alternate timelines like 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 simply devouring them completely. Their victims included the Sixth Doctor, Melanie Bush, Arlene Cole and Stuart Hyde. Each lost a perfect Universe as it collapsed into a nightmarish vision of what could have been, and was saved by Kronos and the Doctor's TARDIS working in concert. Free to act against the Archangel after his release, the Doctor bathed in the Lux Aeterna 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 Mind's computational power and giving the Doctor his chance to convince her to give up the power of the Archangel and return the power of the Lux Aeterna where it belonged. The Archangel did so tearfully and returned to being Angeliqua.