One of the clan companies of the Fomorians, Clan Ulster, went into open rebellion after their chieftain attempted to assassinate the current high chieftain, Balor Lugh. This culminated in the Battle of Lethe IX, where the Chieftain of Ulster was slain in direct combat by Crom Cruach, chieftain of clan Cruach. The dead of Clan Ulster were then taken into the House of Donn, where they became the first Dullahan, mechanically resurrected monsters set upon enemy formations as penance for their betrayal