942. Edmund defeats Idwal of Gwynedd
The Annales Cambriae record that Idwal and his brother Eliseg were killed by the Saxons in 943. (For the re-dating of this annal to 942, and the ordering of the revolt of the Welsh and the retaking of the Five Boroughs, see Beaven, p.7.) The Anglo-Saxon Chronicle makes no mention of this incident, but since Idwal was the king of Gwynedd this must have been a major uprising and Edmund was presumably responsible for crushing it.
M. Beaven, "King Edmund I and the Danes of York", English Historical Review 139 (1918), pp.1-9