Crallo was apparently the son of SS. Sadwrn of Llansadwrn and Canna. He is said to have been a pupil of his uncle, Illtud at Llanilltud Fawr (Llantwit Major), before founding his own religious community at Llangrallo, now Coychurch, in Glamorganshire. It adjoins his mother's foundation at Llangan and he was eventually buried there.