TYSILIO (fl. 600), British saint, was, according to the old lists of saints, the son of Brochwel Ysgythrog, prince of Powys, by his wife Garddun, daughter of King Pabo of the north (Myvyrian Archaiology, 2nd edit. p. 416; Cambro-British Saints, p. 267; Iolo MSS. pp. 104, 130). He founded the church of Meifod, Montgomeryshire, where Beuno is said to have visited him (Life of Beuno in Cambro-British Saints, p. 15). Other churches dedicated to him are Llandysilio, Montgomeryshire, Llandysilio and Bryn Eglwys, Denbighshire, Llandysilio, Anglesey, Llandysilio, Carmarthenshire, Llandysilio Gogo, Cardiganshire, Sellack and Llansilio, Herefordshire. The poet Cynddelw has an ode to Tysilio, printed in the ‘Myvyrian Archaiology’ (2nd edit. pp. 177–9). Professor Rhys regards the name as a compound, of which the first element is the prefix ‘ty-’ seen also in Teilo, Tyfaelog, and Tegai (Archæologia Cambrensis, 5th ser. xii. 37). Tysilio's feast day was 8 Nov.
Tradition makes the saint both a poet and an historian. The ‘Red Book of Hengest’ contains thirty stanzas attributed to him, which are printed in the ‘Myvyrian Archaiology’ (2nd edit. pp. 123–4) and in Skene's ‘Four Ancient Books of Wales’ (ii. 237–41), and are certainly not of the sixth or seventh century. The statement that Tysilio wrote ‘an ecclesiastical history of Britain’ (Pughe, Cambrian Biography) was originally made by Ussher, on grounds which it is now impossible to test (Cambrian Register, i. 26). Nor is it clear what manuscript authority was followed by the editors of the ‘Myvyrian Archaiology’ in styling the first version they print (from Jesus Coll. MS. 28, not, as they state, from the Red Book of Hengest) of Geoffrey's ‘Brut Tysilio’ (2nd edit. p. 432). It appears, however, from a letter of Lewis Morris, printed in vol. ii. of the ‘Cambrian Register’ (p. 489), that a manuscript called ‘Tysilio's History of Great Britain,’ in the handwriting of Gutyn Owain, was in 1745 in the Llannerch collection, and though Morris had ‘never heard of any history written by’ the saint, he at once accepted this as the Welsh original of Geoffrey's history, a view also taken as to ‘Brut Tysilio’ in the ‘Myvyrian Archaiology’ (2nd edit. p. 432) and by Peter Roberts in his ‘Chronicle of the Kings of Britain’ (1811). In point of fact, the ‘Brut Tysilio’ version is a late compilation, of which no manuscript is known of earlier date than the fifteenth century (preface to Rhys and Evans's Bruts, 1890, pp. xvi–xix).[Rees's Welsh Saints, and authorities cited.]