This is often interpreted as an alliance of Northern Brythonic kings, though it is not explicitly stated that they fought together against Hussa. If they did fight together, Rhydderch may have been present at the siege of Ynys Metcaut (the Island of Lindisfarne), where Urien was assassinated by Morcant.

Rhydderch was possibly the leader of the victorious army at the Battle of Arfderydd (dated 573 by the Annals Cambriae).