Complex sentence: Seemingly in shock at seeing the mark of Sigmar on Valten's chest, Archaon was charged by Luthor Huss, who landed a series of blows upon the Everchosen, including almighty blow to the head sending his foe down to one knee.
Split into simpler sentences: Seemingly in shock at seeing the mark of Sigmar on Valten's chest, Archaon left himself open to the charge of Luthor Huss. Huss landed a series of blows upon the Everchosen, including an almighty blow to the head that sent Archaon down to one knee.