Seisyll ap Dyfnwal

Seisyll ap Dyfnwal was a 12th-century Welsh Lord of Gwent Uwchcoed (Upper Gwent).

Family and estates Seisyll was the son of Dyfnwal ap Caradog ap Ynyr Fychan and his wife, said to have been Joyce daughter of Hamelin de Balun. He was a brother-in-law of Rhys ap Gruffydd, the Lord Rhys, King of Deheubarth.

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. on or very near Christmas Day 1175 at Abergavenny Castle. for a banquet by de Braose under a pretext of communication.

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. De Braose later fell from royal favour, dying in exile.