Wikipedia
Tanelorn is a fictional city set in the Multiverse of Michael Moorcock's fantasy novels.
Tanelorn shares some properties with the author's key character, the Eternal Champion, namely its existence in all dimensions, albeit not at the same time (time being a very strange concept in Moorcock's fiction). The city's nature is to appear and disappear both in place and time throughout the multiverse, as it sees fit. Because it can never be destroyed, it is sometimes called The Eternal City. (Its inhabitants, however, may be killed.) In the world of Elric, Moorcock's most famous Champion, the city exists in the Young Kingdoms, somewhere on the fringe of the Sighing Desert. Various characters hint that Tanelorn is accessible from any dimension, to anyone who seeks it, but the search is usually difficult and long. Elric is unique among incarnations of the Eternal Champion in finding the city easily, and in choosing to leave it.
The city serves as a safe harbour for the Eternal Champions and their companions; in the city, they forget for a brief time the burdens their roles lay upon them. Since the city is so closely linked with the Champions, the theme of the championate is ever-present. In one book, inside the bounds of the city are statues of all Eternal Champions, past, present and future, including Erekosë. Tanelorn changes its appearance from one realm to the next.
The city is not allied with either Chaos or Law, but at times it is brought into connection with the Grey Lords (the keepers of balance in Moorcock's Multiverse). Therefore it is prone to attack, especially from the agents of Chaos.
In Moorcock's Swords Trilogy, it is revealed that Kwll, one of the two most powerful gods, is imprisoned in Tanelorn, as it is the only city close enough to hold a being of his power.
It is revealed at the end of the novel The Quest for Tanelorn that the city was made by the Captain of the Dark Ship, and his brother, the Steersman.