Dial Garreg: the Revenge Stone.
The remains of the cross commemorating the murder of Richard de Clare, Marcher Lord, in 1136.
In 1136 Richard had been away from his lordship in the early part of the year. He returned to the borders of Wales via Hereford in the company of Brian Fitz Count, but on their separating, Richard ignored warnings of the danger and pressed on toward Ceredigion with only a small force. He had not gone far when on 15 April he was ambushed and killed by the men of Gwent under Iorwerth ab Owain and his brother Morgan, grandsons of Caradog ap Gruffydd, in a woody tract called "the ill-way of Coed Grano", near Llanthony Abbey, north of Abergavenny. Today the spot is marked by the 'garreg dial' (the stone of revenge). Wikipedia.