The Norman invasion brought with it a set of new surnames, as well as a variation on the patronymic. "Fitz", in modern times considered distinctively Irish, grew out of the French "fils", meaning "son". So, Fitz-Urse would mean "son of URSE".

Reginald Fitz-Urse (born c1150) was the eldest son of Richard Fitz-Urse, on whose death about 1168 he inherited the manor of Williton, Somersetshire; he also held the manor of Barham, Kent, and lands in Northamptonshire.

He was one of the four knights (others being Hugh de Moreville, William de Tracy, and Richard de Breton) who, in 1170, were stirred up by the hasty words of Henry II to plot the archbishop's death. They left Bures, near Bayeux, where the king then was staying, and proceeded by different routes to England, meeting at Saltwood, then held by de Broc.

The next day they set out with a few men.  It is said that the night before the murder they stayed overnight at Barham Court held by Reginald.  Having gathered reinforcements, especially from the abbot of St. Augustine's, at whose house they halted, they entered the archbishop's hall after dinner, probably about 3pm, and demanded to see him.   Reginald told him that he bore a message from the king, and took the most prominent and offensive part in the interview which ensued.  He had been one of Thomas's tenants or men while he was chancellor and the archbishop reminded him of this. This increased his anger, and he called on all who were on the king's side to hinder the archbishop from escaping.

When the knights went out to arm and post their guards, Reginald compelled one of the archbishop's men to fasten his armour, and snatched an axe from a carpenter who was engaged on some repairs.  While Thomas was being forced by his monks to enter the church, the knights entered the cloister, and Reginald was foremost in bursting into the church, shouting "King's men!".  He met the archbishop, and after some words tried to drag him out of the church. Thomas called him "pander", and said that he ought not to touch him, for he owed him fealty.  After the murder had been done the knights rode to Saltwood, glorying, it is said, in their deed, though William de Tracy afterwards declared that they were overwhelmed with a sense of their guilt.  On the 31st they proceeded to South Malling, near Lewes, one of the archiepiscopal manors.

They were excommunicated by the pope, and the king advised them to flee into Scotland.  There, however, the king and people were for hanging them, so they were forced to return into England. They took shelter in Knaresborough, which belonged to Hugh Morville, and remained there a year.

All shunned them and it is said that even dogs refused to eat morsels of their meat. At last they were forced by hunger and misery to give themselves up to the king.  He did not know what to do with them, for as murderers of a priest they were not amenable to lay jurisdiction; so he sent them to the pope, who could inflict no heavier penalty than fasting and banishment to the Holy Land.  Before he left Reginald gave half his manor of Williton to his brother and half to the knights of St. John.  He and his companions are said to have performed their penance in the Black Mountain (various explanations of this name have been given; none are satisfactory; it evidently being intended to indicate a specific place, probably a religious house, near Jerusalem), to have died there, and to have been buried at Jerusalem before the door of the Templers' church.

It was believed that all died within three years of the date of their crime. There are some legends about their fate and it was said that Reginald is said to have gone to Ireland and to have there founded the family of McMahon.