St Deiniol was abbot of Bangor-on-Dee. Many of his monks were killed in the Battle of Chester by the Anglo-Saxon King Aethelfrith because their prayers were deemed potent against his forces.
Troparion
You gathered many monks to Bangor on Dee
and they shone forth as a great light to North Wales;
as Bishop you taught the way of salvation.
Wales still needs your prayers, holy father in Christ!