What is "tathan"

Saint Tathan (also known as Tatheus) is claimed to be a Celtic saint, who travelled from Ireland to Wales where he founded a Christian church. He is reckoned an early abbot of Caerwent and has dedications at Llanvaches, near Caerwent, also known as Llandathan, and at St Athan. He is said to have brought light to the heathens to undo the work of Satan.