Llew Llaw Gyffes’s betrayal, restoration, and slaying of Gronw Pebyr turn a secret entrusted within a marriage into a sequence of downfall, recovery, and retribution. Blodeuwedd and Gronw use Llew’s own account of how he might be killed to arrange the attack; his uncle Gwydion finds him changed into an eagle and restores him, and Llew ultimately reclaims his lands and kills Gronw. The episode’s reversals are bound to precise conditions: the making of a spear, the position of a foot on a buck and the edge of a bath, and, in the final encounter, a stone slab that cannot stop Llew’s dart.

The Secret of Llew’s Death

After Gronw has spent successive nights with Blodeuwedd, the two discuss how they might remain together. Gronw proposes that she learn from Llew, under the appearance of concern, how he could meet his death. When Llew returns home, Blodeuwedd keeps silent at first, then presents herself as sorrowful at the thought of losing him. Her professed solicitude gives her a way to ask for the knowledge that Gronw needs.

Llew explains that he cannot easily be killed. The spear used against him must take a year to make, and work on it can be done only during Sunday sacrifices. He also says that he cannot be slain inside a house or outside one, on horseback or on foot. The apparent impossibility has a narrow exception: he must be in a bath beside a river, under a tightly thatched roof, with one foot on a buck and the other on the edge of the cauldron. Blodeuwedd presents her request as a wish to understand how such a position could be possible; Llew shows her, not knowing that she means to make the conditions actionable.

She sends word to Gronw, who makes the spear and has it ready a year later. She then arranges the scene at the bath and has goats gathered across the river from Bryn Kyvergyr, where Gronw is to lie in ambush. The long making of the weapon and the careful assembly of the setting transform Llew’s supposedly protective explanation into a plan. His confidence in Blodeuwedd makes the disclosure possible; the attack depends on reproducing his description exactly.

The Attack and Gwydion’s Search

Blodeuwedd takes Llew to inspect the prepared bath. He enters, anoints himself, and, at her prompting, asks for one of the animals she calls bucks to be brought. He then steps out and places one foot on the bath’s edge and the other on the buck’s back. From the hill at Bryn Kyvergyr, Gronw rises, kneels, and throws his poisoned spear. It strikes Llew in the side; the shaft comes out, but the dart’s head remains. Llew flies away as an eagle, cries out, and disappears from sight.

Gronw and Blodeuwedd go to the palace together, and Gronw takes possession of Ardudwy. After overcoming the land, he rules both Ardudwy and Penllyn. News of Llew’s disappearance reaches Math son of Mathonwy and Gwydion. Gwydion refuses to rest until he has found news of his nephew, and searches across Gwynedd and Powys before reaching a house in Arvon.

There, Gwydion learns that a sow leaves its sty each day and that no one knows where it goes. He asks to be present when the sty is opened. The sow runs against the course of a river to Nant y Llew and feeds beneath a tree on putrid flesh and vermin. Looking up, Gwydion sees an eagle in the tree; when it shakes itself, the same matter falls from it for the sow to eat. From the eagle’s wounds, Gwydion recognizes Llew. The search reveals a disturbing reversal: the ruler’s nephew survives, but in a changed form, dependent on the sow’s visits for sustenance.

Restoration and the Fate of Blodeuwedd

Gwydion calls to the eagle in song, and it comes down from the tree to his knee. He strikes it with his magic wand, restoring Llew to his own form. The transformation has left Llew emaciated, little more than skin and bone. Gwydion takes him to Caer Dathyl, where physicians from Gwynedd treat him; by the end of the year, Llew is completely healed.

Once recovered, Llew tells Math that he wants retribution from the person who caused his suffering and says that the sooner he regains what is his, the better. The people of Gwynedd are gathered and march to Ardudwy. When Blodeuwedd hears that Gwydion is coming, she flees to the mountain with her maidens. They cross the Cynvael River and head for a court on the mountain, but fear drives them onward facing backward; they fall into the lake. All drown except Blodeuwedd, whom Gwydion catches.

Gwydion does not kill her. Instead, he condemns her to become an owl, never again to show her face in daylight and destined to be attacked and chased by other birds. She retains the name Blodeuwedd, which the tale identifies as the name for an owl in its own time. Her punishment makes the consequences of her betrayal permanent: she survives, but under a form and name that preserve the memory of the wrong done to Llew.

Llew’s Return and Gronw’s Death

Gronw withdraws to Penllyn and sends an embassy asking whether Llew will accept land, territory, gold, or silver in compensation. Llew refuses each offer. The least he will accept, he says, is that Gronw stand where Llew stood when he was struck, while Llew takes aim from the place Gronw occupied. He demands not a negotiated payment but a return to the place and terms of the attack.

Gronw asks his warriors, household, and foster-brothers whether any will take the blow in his place. None agrees. The tale links their refusal to their enduring description as the third disloyal tribe. Gronw therefore goes to the bank of the Cynvael River and takes the position from which he attacked Llew. Llew stands in Gronw’s former place. Gronw asks to put a slab from the riverbank between himself and the dart, appealing to the fact that his deed was accomplished through a woman’s wiles. Llew permits him to use it.

The protection fails: Llew throws the dart, which pierces the slab and passes through Gronw’s back, killing him. The slab remains on the bank in Ardudwy, pierced through, and is called Llech Gronw. Its surviving hole gives the encounter a lasting physical marker, while the reversal of positions completes the repayment Llew demanded. He takes possession of the land a second time, governs prosperously, and, as the story relates, becomes lord over Gwynedd. The ending restores his rule but does not erase the consequences of the conspiracy: Blodeuwedd remains an owl, Gronw is dead, and the pierced slab preserves the place where Llew’s recovery became vengeance.