It's an illuminated manuscript depicting Saint Denis and Saint Piatus, said to have been decapitated for their beliefs.

Statue of St Denis

Edit: comes from this book: https://fr.wikipedia.org/wiki/Livre_d%27images_de_madame_Marie

Maybe they’re martyrs? Sometimes they’re depicted as walking around with their fatal injuries.

https://upload.wikimedia.org/wikipedia/commons/5/58/Altartafel_Kapelle_Spannweid_Z%C3%BCrich_Stadtheilige.jpg

that's Byzantine artwork for ya! here's a post from an art history community that discusses the painting. quite interesting! https://www.reddit.com/r/ArtHistory/comments/e1h0sy/jesus_and_felix_regula_exuperantius_1506/

My thoughts exactly. All I can think of is that religious devotion requires giving away your brain??? It really is a bizarre illustration

I have a theory that initially a lot of these works were made for a laugh, like an old timey joke picture, and over time people kept adding meaning to them.
Like someone makes a rude and crude painting of a giant cock with eyes, makes people laugh, and a few thousand years later some bloke in a trilby goes "hmm, yes, it seems these people worshipped a penis god at this point in time. Also it clearly reads a prophecy, 'deez nuts'. Fascinating."