The kingdom ruled by King Gundebald in Meriadoc.
It was a lake of tar with an island of solid ground in the center. Unwary visitors often sank into the pit, giving the land its name.
King Meriadoc of Wales journeyed to the Land From Which No One Returns and defeated Gundebald there. Meriadoc did, however, manage to escape from this realm, having rescued the daughter of the German Emperor.
Source
Historia Meriadoci Regis Cambrie | Late 13th century