Pwyll Pen Annwn was a prince of the Demetae (Dyfed) tribe (an area covering Pembrokeshire and Carmarthenshire), husband of Rhiannon and father of the hero Pryderi.
Whilst hunting one day Pwyll, after separating from his companions, stumbled across a pack of hounds feeding upon a stag. After driving the hounds away he lets his own hounds feast. However, unbeknown to Pwyll, the hounds were the spectral hounds of Annwn (the otherworld). To repay for his error, Pwyll agreed to take on Arawn's (king of the otherworld’s) appearance and trade places with him for a year and a day.
At the end of the year, Pwyll engages Arawn’s rival Hafgan in single combat. He defeated him with one mortal blow, and gives complete dominion of Annwn. Pwyll and Arawn meet once again and change places. Their friendship is cemented and Arawn rewards him with the title Pwyll, head of Annwn.
One day Pwyll and his nobles noticed the arrival of a beautiful woman dressed in gold silk and riding a white horse. Although Pwyll sent his best horsemen after her, she always remains ahead of them, though her horse only ambles. After three days, Pwyll, rides out but still cannot catch up with her. After calling out to her in desperation, she finally stops and introduces herself as Rhiannon.
She tells Pwyll she was actually seeking him because she is supposed to marry Gwawl ap Clud, but is really wants to marry Pwyll. A year later, as arranged, Pwyll comes to Rhiannon’s kingdom with his soldiers to marry her. He sees a distraught man, who asks for a favour of Pwyll. Pwyll rashly promises that he will grant whatever the man asks. However, the man is Gwawl and he asks for Rhiannon’s hand in marriage. Pwyll has no choice and another wedding is planned another years hence.
Rhiannon devises a plan and one year later Pwyll enters the pre wedding festivities dressed as a beggar. He asks Gwawl for a bag full of food. Gwawl agrees, but Pwyll places Gwawl inside the bag. Pwyll and he and his men beat him brutally. Gwawl submits and asks Pwyll to spare his life. Pwyll agrees and finally wins the hand of Rhiannon through deceiving and dishonoring of Gwawl.