Serra's Realm

Serra's Realm was an artificial 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.

Geography

Serra's Realm was a plane where the sky was in perpetual dawn. The sunrise came from various angles at once, and it was impossible to tell one's position. Landmasses floated throughout the sky, carrying large meadows with boulders and berry-producing vegetation. These flying lands never collided with each other, each seemingly to move along a fixed path. It was also impossible to reach the end of such a landmass on foot, because, like Mercadia, the laws of physics had been altered and moving in one direction to reach the end of the floating island would result in one's repositioning back where he or she started.

There were cities scattered here and there, created and supported by Serra.

Serra's Sanctum was a notable landmark of the plane, suspended somewhere in its infinite sky. The Sanctum was the dwelling place of the famed planeswalker herself. Powerful artifacts run through white mana and having white-related functions, such as the Cocoon, could be found here.