Ælla became king after Osberht (Osbryht) was deposed. The beginning of his reign is traditionally dated to 862 or 863 but evidence about Northumbrian royal chronology is unreliable prior to 867. His reign may have begun as late as 866. Almost nothing is known of Ælla's reign; Symeon of Durham states that Ælla had seized lands at Billingham, Ileclif, Wigeclif, and Crece, which belonged to the church. While Ælla is described in most sources as a tyrant and an illegitimate king, one source states that he was Osberht's brother.