Complex sentence: Edgar received limited support from William Rufus as Donnchad had before him; however, the English king was occupied with a revolt led by Robert de Mowbray, Earl of Northumbria, who appears to have had the support of Domnall and Edmund.
Split into simpler sentences: Edgar received limited support from William Rufus as Donnchad had before him. However, the English king was occupied with a revolt led by Robert de Mowbray, Earl of Northumbria, who appears to have the support of Domnall and Edmund.