Gwyddno Garanhir was the supposed ruler of a sunken land off the coast of Wales, known as Cantre'r Gwaelod. He was the father of Elffin ap Gwyddno, the foster-father of the famous Welsh poet, Taliesin, in the legendary account given in the late medieval Chwedl Taliesin (Ystoria Taliesin/Hanes Taliesin; "The Tale of Taliesin").