The prince of Dyfed who swapped places with Arawn in the Mabinogion is Pwyll. Their exchange is the central episode of the First Branch of the Mabinogi, often titled Pwyll, Prince of Dyfed.
While hunting at Glyn Cuch, Pwyll drives away Arawn’s hounds from a stag they have killed and lets his own dogs feed on it. Arawn, ruler of Annwn—the Welsh Otherworld—appears and rebukes him. To make amends, Pwyll agrees to take Arawn’s form and rule Annwn for one year and one day.
During the exchange, Arawn rules Dyfed in Pwyll’s appearance, while Pwyll faces Arawn’s rival Hafgan. Arawn instructs him to strike Hafgan only once, because a second blow would allow the enemy to recover. Pwyll follows the instruction and defeats him, earning Arawn’s friendship and the title Lord of Annwn.
Pwyll later returns to Dyfed and encounters Rhiannon, who becomes his wife. Pryderi, their son, becomes one of the major figures of the later branches of the Mabinogi.