The Otherworld consists of different segments: Underworld, Afterworld, various isles and planes (see other isles and planes). The Otherworld is only referred to such on general terms, but is otherwise referred to as various names depending on where you derive from (see other names). The Otherworld is most commonly referred to as Ossian in the North West, Tir na nÓg Quarter. To the Milesian human Outerworld it is known as the 'Land of the Dead' -- little is known of its existence, other than that of mythological mentions during the time that Otherworld species (see species) inhabited the Outerworld thousands of years ago. The Otherworld cannot be seen or visited by humans. The plane was created as a perpetual retreat during the aftermath of the Conquest of the Milesians. It contains thousands of isles, land masses, rivers, seas and mountains; some still uninhabited, but mostly inhabited by a patchwork of different species, races and houses-- all with different rulings, courts and doctrine.
The creation of The Otherworld has been widely credited to the great draiochta blood warrior Oisin. He is a mythological figure and no one knows of his existence, other than that of tales and scrolls. It is said he helped construct the original plane of Otherworld to which it only featured Tir na nÓg Quarter, with the help of his wife Niamh Óir. The original purpose of the plane was initially a love sanctuary between that and his wife; inherited by his great grand-daughter Plor, daughter of Plúr Na, this was eventually given over to the original House Findias who helped construct it further as a perpetual retreat during the Conquest of the Milesians.
An Egress is the only medium between the Otherworld and Outerworld and can only be harnessed by an individual who can hinder draíochta; however exceptions have been known. The Otherworld plane is somewhat parallel to the human Otherworld, however time lagging is a prominent characteristic of travelling between these two planes and in some cases people have been reported to be gone for hundreds of years when returning to either plane.