Mayisha is located in a different galaxy than Earth. Nothing has been revealed about its physical details, but its primary inhabitants are--or rather, were--technologically and magically advanced humanoids, as well as the undead.
According to Thievry, Mayishans are the ancestors of humans.
The planet had a mostly peaceful history--for the living Mayishans, anyway; the undead were outcasts. In the year 7 B.C., they rebelled against the living Mayishans, led by then-princess Quita. Her sister Ellikacha was able to seal the rebels away, killing many, but the power killed every other Mayishan on the planet. Ellikacha fled to Earth. The planet's fate since then is unknown.