He held lands in present-day Monmouthshire, part of the old Welsh Kingdom of Gwent, and his main base was at Castell Arnallt, a motte and bailey style fortified site situated near the River Usk a few miles south of Abergavenny, near modern-day Llanover.
Seisyll ap Dyfnwal is best known for being an unwitting victim of the Norman Baron, William de Braose, 4th Lord of Bramber, who had him killed[2] on or very near Christmas Day 1175 at Abergavenny Castle.
[3][4] Seisyll, his eldest son Geoffrey, along with other Welsh princes and leaders from the area, were invited to Abergavenny Castle[5] for a banquet[6] by de Braose under a pretext of communication.
[5][3][1] De Braose and his men then mounted horses and galloped the few miles to Seisyll's home where they killed his seven year old younger son, Cadwalladr, and captured his wife, whose exact fate is uncertain.
[3][5][1] De Braose's strategy was to eliminate all those who could have done it and effectively remove the experienced leadership of the Welsh forces in the area, destabilising the region and seize the opportunity to gain the upper hand.