Very little is known about the 3rd century Christian martyrs Denis, Rusticus, and Eleutherius. The three were, most likely, Romans and Denis was a friend of Pope Fabian (unknown-250).
Fabian knew that Denis was a good priest and he sent him to Gaul. Denis was almost always accompanied by Rusticus, another priest, and the deacon Elutherius. Shortly after their arrival, Denis became the bishop of modern day Paris. The suburb he occupied is now known as the commune of Saint-Denis in Paris. The Cathedral Basilica of Saint Denis can still be seen to this day.
The lives of the three holy men led to a shocking number of Christian converts. It did not take long for them to become the mortal enemies of the local pagan priests. On an unknown date, the three were captured and tortured horribly. They were then taken to the top of Montmartre, the highest hill in Paris, and beheaded. Although the year of their martyrdom is unknown, it is thought to have been sometime between 250 and 258 A.D.
In ancient times, Montmartre had been a druidic holy place. At the time of Saint Denis, it would have been known as Mons Martis or “the Mount of Mars”. After the story of Saint Denis and his companions spread, the hill became known as Montmartre or “the Mountain of the Martyr”.
There is a legend that, after the pagans chopped off Denis’ head, he picked it up and walked for at least six miles. He is, therefore, considered a cephalophore which is Greek for “head carrier”. Although there is absolutely no evidence to prove that this actually happened, Saint Denis is almost always depicted in religious artwork as a bishop carrying his severed head.
There have been several other cephalophore saints; however, nearly all of them died before the 11th century and, consequently, there is very little evidence of this phenomenon that can be considered trustworthy.
Because there is so little known about Saints Rusticus and Elutherius, they are usually overlooked. Denis, however, is considered the patron saint of Paris and of those who are diabolically possessed. He is also one of the Fourteen Holy Helpers who were invoked during the 14th century.
The memorial of Saint Denis and his companions is on October 9.
Source: EWTN online article “Saint Denis”