The cauldron's origin is explained during the negotiations between Bran and Matholwch. Its former owners are Llasar Llaes Gyfnewid and Cymydei Cymeinfoll, extraordinary figures associated with a lake in Ireland. Their presence becomes intolerable to the Irish, who attempt to destroy them in an iron house heated around its occupants. The couple escapes and reaches Britain, where Bran receives them. In return for his protection they give him the cauldron. The story thus places the vessel within a history of hospitality, violence and movement between the two islands before it enters the marriage alliance.

Matholwch comes to Britain to marry Bran's sister Branwen. Her half-brother Efnysien, also commonly spelled Efnisien, reacts angrily to an arrangement made without consulting him and mutilates the Irish king's horses. Bran offers compensation and then adds the cauldron to restore goodwill. It is therefore more than an ordinary wedding present: it is a diplomatic gift intended to repair an injury. Matholwch returns to Ireland with Branwen, but resentment survives the settlement. Her subsequent mistreatment and the message she sends through a trained starling eventually bring Bran and his army across the sea.

A proposed reconciliation in Ireland fails disastrously. Armed men concealed in bags within a great hall form part of a planned ambush, and Efnysien discovers them. He then commits a further terrible act by throwing Branwen's son Gwern into the fire. Battle follows. The Irish use the cauldron to recover their fallen warriors: a dead man placed in it returns the next day as before, except that he cannot speak. The tale does not describe these men as invulnerable, nor does it require the modern idea of an automatically replenishing zombie army. Their restoration nevertheless gives the Irish a devastating advantage.

Efnysien recognises the catastrophe surrounding him and conceals himself among the Irish dead. When he is thrown into the cauldron, he stretches his body until the vessel breaks into four pieces and his own heart breaks with the effort. His destruction of the weapon does not turn the war into a simple triumph for Bran's people. Only seven of the invading men survive, Bran receives a mortal wound, and Branwen later dies after returning to Wales. The cauldron's end belongs to the tale's broader destruction of families and communities, even though Efnysien's final act prevents further use of its power.

Welsh literature contains other remarkable vessels. Preiddeu Annwn describes a cauldron in an Otherworld expedition involving Arthur; Culhwch and Olwen includes the seizure of Diwrnach's cauldron; and Ceridwen's vessel belongs to the story of inspiration and transformation. Similarities invite comparison but do not make them the same object. Theories connecting Celtic vessels with the Holy Grail should likewise be treated as interpretations, not an established line of descent. Later adaptations, including Lloyd Alexander's The Black Cauldron, reshape the material for new audiences. Pair Dadeni remains distinctive for its restoration of life without speech and its destruction through a fatal act from within.