Zorsarcos
Zorsarcos, known as the Planet of Serenity, is a largely undisturbed planet known to few beings that are from off-world. A wide oblate spheroid, its equatorial radius is roughly 2.5 times that of its polar radius. On its north pole, a vast tree known as the cosmic tree of Zorsarcos (simply the cosmic tree to zorsarcosians) dominates the horizon, large enough that it casts a shadow over the northern hemisphere and is easily visible from space. Most of Zorsarcos is an ocean world, but there is a large continent below the tree, covered in dense, hospitable rainforest. The dominant species on Zorsarcos is the Zorsarcosian siren, a creature that resembles a bird or fish that has the breast and head of a human. The exact nature of the siren's anatomy is unknown, but their structure is said to vary considerably. Some are almost identical to mermaids, where others more closely resemble harpies, and still others have some variation on this - there are mentions in various texts of those with a fish's tail and bird's wings, with the lower body of a fish and bird's legs, the body of a bird, no legs and fish scales instead of feathers, and so on. They are also said to travel in groups that share the same anatomy, though they seemingly treat each equally regardless.
The civilisation of Zorsarcos is largely mysterious. The sirens do have underwater cities and cities beneath the tree, some of which are vast and rich in resources, but very few reports exist of the precise details of such places. It is known, however, that the sirens do not have formal governments, and their society revolves largely around medical care. They speak a language known simply as "siren" and no other languages.
There are many wild beasts that live in the forests of Zorsarcos, including enormous hydras, scyllas and living whirlpools, but these are largely avoidable. The primary reason that Zorsarcos is poorly understood is that of the few magicians that both know about it and have the wherewithal to travel there, almost none return.