: UHF-irn or IHF-irn
Generally translated as "Hell," this place--particularly in the writings of Taliesin--is essentially a cognate of "inferno"--the lake of fire.
It is said in some places that the king of Uffern is Arawn, who is otherwise called the lord of Annwfn in Pwyll pendeuc Dyfed. Elsewhere, Nudd is said to be king, or his son Gwyn ap Nudd; Stephen Lawhead makes use of this in his Pendragon series. W. F. Skene, in his notes on The Four Ancient Books of Wales, makes an argument that Uffern was originally a name for the part of Scotland ruled by Arawn/Augustus, uncle of Arthur and brother of Urien. This is unlikely.
It is thought that Uffern took on the role that Annwfn/Annwn once played, that of the pagan Otherworld, while also playing the role of the Underworld. In earlier times, they were two distinct ideas; indeed, it is difficult to say if the early Welsh had any concept of a hell under the Druids, who taught reincarnation; as said before, "uffern" is a borrowing of the Latin "inferno." "Hell" is Norse.