According to Lewis Spence (in the Encyclopedia of Occultism,
1920), Jean was a French magician, votary of Apollonius of
Tyana. He traveled from town to town, wearing an iron collar
and making his living by performing deeds of charlatanry. At
Lyons he attained some measure of fame by his miraculous
cures and met with the sovereign, to whom he presented a magnificent
enchanted sword. In battle this weapon became surrounded
by ninescore drawn knives. Jean also gave this prince
a shield containing a magic mirror that divulged the greatest
secrets. The arms vanished or were stolen.
(Unfortunately, Spence did not state the period or the ruler
involved, but this is probably a medieval legend.)