The Well of Voyage.
The Well of Voyage is a well that allows transport between Tirannwn and Kandarin, located in the Underground Pass in Iban's chamber after he is defeated. It was constructed by the elves in the Second Age as a means of expanding their land, but it was captured recently by Iban and cannot be used even after completion of the Underground Pass quest. Regicide must be started for the player to go through.
If you examine it outside before you go in it says: The Well of the Damned.