This T. H. Parry-Williams Memorial Lecture was delivered at the National Eisteddfod in Anglesey in 2017. Owain Lawgoch or Owain ap Thomas ap Rhodri was the grandson of Rhodri ap Gruffudd, the youngest brother of Llywelyn and Dafydd, the last two Princes of Wales and was the last heir of the Gwynedd dynasty. Assassinated in France by an English agent in 1378, few in Wales would have seen him and it is therefore not entirely surprising that he joined the ranks of the Sleeping Heroes, those charismatic figures whose deaths could not easily be accepted and who were believed to be awaiting the call to return and restore the dignity of their peoples. These heroes included Arthur and Owain Glyn Dŵr in Wales, Frederick Barbarossa in Germany and Dom Sebastian, King of Portugal, killed in an attack on Morocco in 1578. Indeed, this belief in the return of a hero is known as Sebastianism and it also came to be tied up with the teaching of the twelfth-century Cistercian abbot Joachim of Fiore (1132–1202).