The Three Harmful Blows of the Island of Britain:

• The first of them Matholwch the Irishman struck upon Branwen daughter of Llyr
• The second Gwenhwyfach struck upon Gwynhwyfar: and for that cause there took place afterwards the Battle of Camlann
• And the third Colydan the Poet struck upon Cadwaladr the Blessed.

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Of the first, this is recounted in The Mabiogion section called "Branwen uerch Llyr"; it is because of this, that the battle of Ireland was fought, killing Bran.

Of the second, this is a tradition I've only ever seen in the triads, that the Battle of Camlan was related to a fight between Guenevere and another woman, and not related to Mordred. Curious...

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