The Christmas Massacre of 1175
In the 1160s, Henry Fitzmiles, 1st Earl of Hereford and lord of Abergavenny was murdered reputedly by Seisyll ap Dyfnwal of Castell Arnalt. Henry had no male heir; thus, Abergavenny Castle passed to his daughter Bertha's husband, William de Braose. It was a time of tension between Welsh Princes and lords in Wales.
Christmas is a traditional time for settling differences under the pretense of peace and starting new eras among the Welsh. Thus in 1175, William de Braose invited Seisyll ap Dyfnwal and other Welsh noblemen of Gwent to Abergavenny Castle at Christmas, supposedly as an act of reconciliation and with the understanding that they could voice their grievances, overcome differences, and begin a new period of peace, following a period of conflict.
Some Welsh noblemen did not trust de Braose and declined the offer. But Seisyll and his eldest son Geoffrey attended the dinner, and other Welsh nobles followed Seisyll's lead and also attended.
When they arrived at the Abergavenny Castle, they were greeted and told to surrender their weapons before entering the Great Hall to ensure the peaceful intent of all involved. Once inside, the doors were locked, and armed men cut them down without mercy. De Braose and his men then mounted their horses and rode the few miles to Seisyll's home, where they murdered his seven-year-old son, Cadwalladr, and captured his wife.
De Braose's vicious act was in retribution for the murder of his uncle in the 1160s. De Braose earned the nickname the "Ogre of Abergavenny" for his conduct, sanctions were levied against him, and he was "retired" from public life resulting in the castle being passed to his son. Siesyll's death was later avenged in 1182 by Hywel ap Lorwerth, lord of Caerleon, in a campaign where Abergavenny Castle was stormed and set ablaze.