Serra's Realm

Serra's Realm was a created plane, created by the planeswalker Serra from pure white mana as a heavenly ideal. Serra made it in the hopes that she could make a haven where suffering could be avoided forever. It was the original home of the famous Serra Angels, and existed in peace and worship of its creator for some time.

All of that changed when Urza fled there after his failed attack on Phyrexia. He spent five years healing from that attack, and learning about the nature of artificial planes, which require constant maintenance from a planeswalker or other powerful being to keep from being consumed by entropy. After Urza left, the Phyrexians attacked. Though they were repulsed, their black mana forever tainted the realm, causing Serra to flee.

After Serra left, her head archangel, Radiant, took control of the plane with increasing militancy and religious paranoia. In her quest to expunge all Phyrexian influence, Radiant became an unwitting tool of the Phyrexians, and began exterminating loyal Serrans. Urza returned to Serra's Realm with the newly-built Skyship Weatherlight, rescuing many Serran refugees before finally collapsing the plane into the ship's powerstone core, giving it enough power to travel the planes.

At some point after the destruction of the Weatherlight, Serra's Realm may have come back into existence in some form.