Navarre spent the latter half of the 14th century fouling up alignments in the Hundred Years’ War by constantly switching his allegiances between England and France. Come the 1370s, he was supposed to be on team France — having paid homage to the French king in 1371 — but was still conniving with the English whose expeditions might one day apply enough pressure to force France to restore him some lost domains.
The last great plot of the man contemporaries knew as Charles the Bad really fell apart in the spring of 1378 when the French detained en route to Normandy Jacques de Rue and Pierre du Tertre, two emissaries of Charles’s “criminal entourage”. They carried coded messages** confirming that Navarre was not only back to scheming with the English, but that he was trying to orchestrate the assassination of the French king by means of poison — plots that Jacques confirmed under torture.
France retaliated by attacking its disloyal partner’s Norman holdings and by year’s end the whole region had been chopped up between the French and the English, never to return to Navarrese hands. His retainers were put to death and their corpses strung up on Montfaucon.
This was the humiliating end to the political life of Charles the Bad: reduced to a client king dominated by France (to his north) and Castile (to his south). It would soon find its parallel in the horror ending of his actual life on New Year’s Day 1387:
Charles the Bad, having fallen into such a state of decay that he could not make use of his limbs, consulted his physician, who ordered him to be wrapped up from head to foot, in a linen cloth impregnated with brandy, so that he might be inclosed in it to the very neck as in a sack. It was night when this remedy was administered. One of the female attendants of the palace, charged to sew up the cloth that contained the patient, having come to the neck, the fixed point where she was to finish her seam, made a knot according to custom; but as there was still remaining an end of thread, instead of cutting it as usual with scissors, she had recourse to the candle, which immediately set fire to the whole cloth. Being terrified, she ran away, and abandoned the king, who was thus burnt alive in his own palace.
* There are some cites for May 21 out there, but the sourcing on June appears stronger to me, and references to the men’s interrogations and trial run to June. The beheading is also referred to as having taken place on a Monday, which fits June 21 (but not May 21) in 1378.
** According to CryptoSchool this is one of the oldest known documents in the history of cryptology. Devised personally by Charles of Navarre, its gambit was to “move the names of princes, castles and cities to other names not their own.” (Chronique Normande)