Complex sentence: Drustan decides that three thousand foot soldiers and four hundred horsemen will journey to Terre d'Ange to aid Ysandre, with the rest left behind to secure Alba; the Twins decide that they will go along as well, with half of their fighting force.
Split into simpler sentences: Drustan decides that three thousand foot soldiers and four hundred horsemen will journey to Terre d'Ange to aid Ysandre, with the rest left behind to secure Alba. The Dalriada decide that they will go along as well, with half of their fighting force.