Tewdwr Mawr (Breton for "Theodore the Great";[1] Cornish: Teudar Maur or Teudaric; Welsh: Tewdr; Latin: Theodorus; French: Thierry; mid-6th century) was an early medieval king in Armorica (now Cornouaille, France) and Cornwall.
While Tewdwr was still young, his grandfather Budic II was overthrown and forced into exile at the court of Aergol Lawhir of Dyfed.
[1] He opposed Breage's mission (although Sabine Baring-Gould placed its arrival around 500[2]), first compelling them to land at Reyvier instead of Carnsew[2] and then later martyring several of its members, including Ia of Cornwall; Saint Gwinear met with a similar fate, being thrown with his followers into a pit of reptiles.
After decades of exile, Tewdwr returned to Brittany and defeated Macliau and his oldest son Jacob in 577.
[1] Having made martyrs of the Irish missionaries he opposed, Tewdwr became so infamous for his oppression that he began to appear in the hagiographies of saints he could not possibly have known, such as the 4th-century Meriasek.