The Massacre at Abergavenny, Wales, Christmas 1175

William de Braose invited Seisyll ap Dyfnwal and some other local Welsh leaders over for Christmas dinner, at which they were all going to agree to live in peace and whatnot.  This made sense to the Welsh, who normally wouldn't have trusted William de Braose any further than they could throw him, because for them, it was the time of reconciliation!  Settling debts! Being nice!  So you can imagine what a shock it was when William had the doors shut and murdered everybody.  Then, because he wasn't done yet, he went on over to Seisyll's castle, captured his wife Gwladys, and slaughtered his 7 year old son Cadwaladr. Merry Christmas!  Naturally, the Welsh never forgot this. Anglo-Norman and Welsh relations were set back for decades, and they hadn't been good to begin with. (Bonus!  Popper the parrot decides to chime in.)