Annwn

Annwn is at the source of all worlds, where the primal forces of life exist and where the dead must fall to decompose and renew the earth. The size of this world is limitless in comparison with the other two, Gwynfyd and Abred.
Two Annwns co-exist in the depths of the earth, one magical and divine and the other more physical in nature. The Under-Earth of Annwn is a place of wonder and dangers, just as much as those on the surface, if not more so. This dark dismal realm is misunderstood by many beings, who perceive it only to be of darkness and evil beings, but in fact it is so much more on many levels.
Although vast portions of Annwn are dark and unlit, there are many areas where light does exist in a myriad of forms, and entire worlds comprised of beings with their own lives and histories, completely independent of those found in the surface worlds. The true nature of Annwn is not understood by many; it is not simply the underfoot of the earth. The near endless scores of inhabitants that live here are called the Plant Annwn or “Children of Annwn” who comprise the Ghost Elves and all other races.
Billions of miles of corridors, caverns, worlds and more are found here and could never be fully explored or comprehended. Just like with Faery, the Three Powers share this world uneasily. The ruling Dark Fey are the accursed Ysgodiau led by the Pen Annwn in the Twelve Kingdoms, whilst there are very few substantial realms of powerful Light Fey to counter them and keep them in check, placing Annwn in an unbalance almost constantly.
Terrible horrors and monsters dwell here in the ghost-haunted tunnels. Equally there are splendid wonders and unimaginable wealth in the deep earth that no adventurer could turn away from. Everything ever buried and forgotten finds its way to Annwn over time, sometimes via interesting paths.
Treasures, monsters and plentiful adventures can be found here easily. There is no end to the wonder, horror and discoveries in this underworld setting. It can open out into either Faery or the Mortal world seemingly at random since it underlies both realms.
Annwn is an unstable realm that often has quakes and tremors, usually as a result of magic being used. Travel here is dangerous to all, natives or strangers, because of this fragility.
The few brave Faery and Mortals from the surface worlds that have attempted to raid here and harry the Pen Annwn or other threats rarely leave the way they came. Arthur and his greatest champions have plundered Annwn but not without great loss to men and sanity. This world is not for the foolhardy or greedy; it will devour the weak and incorporate them to its spectral whole. Flickering ghosts travel and wail through the many tunnels and caves, finding their way to the next life or to one of the many gods of the underworld. The reverberating echoes from mysterious beasts and monsters howl in distant areas of the caverns at random times. It is always ominous and tense whilst traveling in the Under-Earth. It is possible, though tedious and unlikely, that travelers in Annwn can journey and find an exit out of any cave, grave, basement, mine shaft, dungeon or any other subterranean level in the surface worlds.
Oceans and rivers exist deep in Annwn just as they do above ground, and the animals found here are ghostly white. The Cwn Annwn come from this world, and dwell in the Kingdom of the Pen Annwn. To have them above ground under one’s control is a rare privilege.
Druids speak of a secret chamber somewhere in Annwn where the books written by the gods lie. The many secrets, spells and histories are constantly updated in these tomes. They have never been found or seen by prying eyes and it is hoped they never will be.