Not only human dead are crafted into servants of the necromancers. Beast of burden are rare in the caverns, so the horses Agartha can get their hold on are used and reused in work and in war. Dead horses are quicker than humans, and can carry more. The Ktonian Necromancers have created horrible iron reinforced skeletal horses with Cave Fire barrels placed inside their chests. The nightmares are not very good at combat, but as carriers of the alchemical load they are superior to humans.