Also known as Denys and Dionysius.

Saint Denis was the first Bishop of Paris. He was imprisoned during the persecutions of Emperor Decius and martyred in Montmarte, France. His remains were tossed into the Seine and were pulled from the river by some of his devoted converts. Tradition has it that Denis was tortured and beheaded and that his headless body carried his severed head away from the execution site. For this reason, Saint Denis is commonly invoked against headaches.

 The Catholic Encyclopedia