Sellack is a village in the Wye Valley, 3 miles NW of Ross-on-Wye. The village is a small settlement on the S bank of the river, consisting of a few houses and the church. St Tysilio’s has a narrow nave and chancel in one; the nave with a S porch and a 3-bay N aisle. The W bay of the nave arcade is 12thc, while the other two bays open onto a broad transept. This was added in the 14thc and rebuilt in a restoration by George Pearson of 1841-42. There is a 13thc N chapel, entered from the chancel and the nave aisle through a broad 13thc arches, and a modern vestry to the E of the chapel. The only Romanesque feature is the W bay of the N arcade.

St Tysilio, or Tesiliog, or Suluc, was a 7thc Prince of Powys who left his family to become a monk at Meifod, Montgomery, later forming his own community on an island in the Menai straits now called Ynys Tysilio. He is said to have fled to Britanny in response to pressure to take up his family duties, and to have built a chapel at Saint-Suliac, where he lived until his death.

Sellack is not recorded by name in the Domesday Survey, but between 1291 and 1351 the church was known as Baysham church (Baysham is a small settlement of a few houses within Sellack parish). In 1086 Baysham was held by Walter from William fitzNorman, and it had been held by Merewine from Edward the Confessor before the Conquest. Being in Archenfield it was not assessed in hides, but with 2 ploughs in demesne and a further 14 men with 7 ploughs it was a good-sized settlement.

The RCHME refers to the church as St Tesiliog. The dedication (under any spelling) is said by Pevsner (1963) to be unique in England although there are churches dedicated to him in Wales and Britanny.