Hafgan first matters to the story through Arawn's account of their rivalry. Arawn identifies him as another king of Annwn whose dominions lie opposite his own and who continually makes war upon him. On the mortal side, Pwyll is hunting in Glyn Cuch when he drives strange white hounds with red ears away from a stag and lets his own dogs feed. Their owner is Arawn. Pwyll's offence against the king's hunting rights becomes the reason for a bargain that will bring the prince into Hafgan's conflict.

Arawn offers to place his appearance upon Pwyll and send him to govern Annwn for a year. He will meanwhile take Pwyll's form and rule Dyfed, so neither household recognises the exchange. A meeting with Hafgan has already been arranged at a ford at the year's end. Before they part, Arawn tells Pwyll to strike his opponent once and refuse any request for another stroke. His explanation is practical within the tale's supernatural world: after an earlier second blow, Hafgan fought him the next day as well as before. It is not presented as a general rule governing every inhabitant of Annwn.

At the appointed time, Pwyll arrives in Arawn's likeness with the nobles of his borrowed kingdom. A herald directs the assembled followers to stand aside because the dispute belongs to the two rulers. The combat is brief. Pwyll's first thrust strikes the centre of Hafgan's shield, splitting it and breaking his armour. Hafgan is thrown beyond the back of his horse and receives a mortal wound. The violence of this single encounter contrasts with the prolonged rivalry described at the beginning: Pwyll succeeds where Arawn's earlier handling of the duel had failed.

Hafgan addresses the man he believes to be Arawn and challenges his right to kill him, saying that he has done him no injury. He then asks for the work of killing to be completed. Pwyll refuses, keeping the instruction he was given rather than accepting his opponent's request. Hafgan calls upon his followers to carry him away and tells them that he can no longer protect them. The narrative treats the wound as fatal but does not linger over a separate deathbed scene. Its attention turns immediately to the submission of his territory.

Still acting as Arawn, Pwyll accepts homage and declares that willing submission will be received kindly, while resistance will be overcome by force. By noon the next day, the two kingdoms are in his power. He then meets Arawn again at Glyn Cuch, and each resumes his own appearance and household. Their friendship continues through gifts and reciprocal goodwill. Pwyll becomes known as Pwyll Pen Annwn, Head of Annwn, because of his year of rule and the victory that united the kingdoms. Hafgan's role ends with the defeat: the First Branch supplies no later revenge, successor or further adventure for him.