Complex sentence: Sigurd the Stout, the Earl of Orkney, was offered command of all the Norse forces, while the High King had sought assistance from the Albannaich, who were led by Domhnall Mac Eiminn Mac Cainnich, Mormaer of Ce (Marr & Buchan).
Split into simpler sentences: Sigurd the Stout, the Earl of Orkney, was offered command of all the Norse forces. Likewise, the High King had sought assistance from the Albannaich, who were led by Domhnall Mac Eiminn Mac Cainnich, Mormaer of Ce (Marr & Buchan).