He was the son of the Queen of Wales and an incubus. He is an incubus hybrid who served King Uther Pendragon. Uther loved Igraine, the wife of Duke Cornwall, so he secured Merlin's assistance in winning her love at the price of leaving his child in Merlin's care. Merlin told Uther a prophecy about his child, who would become his appointed successor during their war-torn times.

(Source: Type-Moon Wiki)