Grugyn enters the story as one of the seven young boars accompanying Twrch Trwyth. Arthur explains that the great boar was once a king transformed for his wickedness, while Grugyn's own reply describes the brood's altered condition as suffering. The narrative supplies no individual childhood or account of Grugyn's former human life. His role is bound to the impossible tasks set by Ysbaddaden for Culhwch, who seeks to marry Olwen. The comb, razor and shears carried by Twrch Trwyth become the objects of an expedition involving Arthur's warriors, extraordinary hounds and carefully specified equipment.

Menw first investigates whether the treasures are present, but the parley conducted by Gwrhyr follows fighting in Ireland rather than occurring before any blood has been shed. Gwrhyr Gwalstawd Ieithoedd, the interpreter who can communicate with the beasts, approaches in the shape of a bird. Grugyn answers for the group. His silver bristles distinguish him visually as he rejects Arthur's demand and declares that his father will not yield the treasures while alive. His threat to cross the sea turns the attempted negotiation into a warning of further devastation. The boar is not a silent obstacle: the encounter gives the pursued creatures a voice.

The boars cross to Wales, landing at Porth Clais in Dyfed, while Arthur follows in Prydwen with his company. At Dyffryn Llychwr, Grugyn and Llwydawg turn upon the huntsmen, and only one escapes. The king brings the hounds against them, and Twrch Trwyth returns to their defence. As the pursuit continues through the Amman country, the other young boars are killed. The tale names several of them, including Twrch Llawin, Gwys, Banw and Benwig. Eventually only Grugyn and Llwydawg remain beside their father. Their survival at this stage makes the later separation an important turning point in the chase.

At Llwch Tawe, Grugyn parts from the others and makes towards Din Tywi and Ceredigion. Eli, Trachmyr and a larger company pursue him. His route ends at Garth Grugyn, where he is slain after killing Rhuddfwy Rhys and others among his pursuers. This last stand belongs to Grugyn, not to a successful defeat of Twrch Trwyth. The place-name links the animal with the landscape of the story, though identifying its precise modern location is a separate geographical question. The narrative records the deaths without requiring an invented account of every weapon or movement in the fight.

Llwydawg's final battle follows at Ystrad Yw, where men from Brittany encounter him; the presence of those men does not move the battle itself to Brittany. Arthur's company then confronts Twrch Trwyth in the Severn, where the razor and shears are taken. The comb is obtained later in Cornwall before the great boar is driven into the sea. These stages matter because Grugyn's death is one loss within a longer pursuit, not the hunt's conclusion. What distinguishes him is the gleam of his bristles and his refusal to surrender, a brief but memorable challenge to the power assembled against his family.