Matholwch, King of Ireland, is a central character in the Second Branch of the Mabinogi, the tale of Branwen ferch Llŷr. He initially seeks the hand of Bran the Blessed’s sister, Branwen, in marriage, which Bran graciously agrees to. However, tensions arise when Efnisien, Branwen’s half-brother, mutilates Matholwch’s horses out of jealousy and anger. Despite this offense, Bran reconciles with Matholwch by giving him the magical cauldron of rebirth, Pair Dadeni.

After their marriage, Matholwch mistreats Branwen, forcing her into servitude. Branwen, in distress, sends a message to Bran via a starling, pleading for his help. Bran sets out for Ireland to rescue her, and upon his arrival, Matholwch offers peace and builds a house to accommodate the giant.

However, tensions escalate when Matholwch’s subjects object to his plan to pass the kingdom to his son by Branwen, Gwern. In the ensuing conflict, Efnisien, realizing the treachery, kills the Irish lords hiding in flour sacks. He also throws Gwern into the fire, sparking a devastating war that leads to the deaths of many, including Branwen.

Ultimately, only a handful of Welsh survivors return home with Branwen’s body and the severed head of Bran the Blessed, as the tale ends in tragedy, with Branwen dying of grief upon seeing the destruction caused by the war.