It is one of the Three Grinding Gashers, Glas, Glessic and Gleisad. They are worn by the three sons of Cleddif Kyfwlch, in the Arthurian story of Culwich and Olwen
Bwlch, and Kyfwlch, and Sefwlch, the sons of Cleddyf Kyfwlch, the grandsons of Cleddyf Difwlch. (Their three shields were three gleaming glitterers; their three spears were three pointed piercers; their three swords were three grinding gashers; Glas, Glessic, and Gleisad.)
Glessic is also the name of one of the domains in Kingdoms of Camelot.