In this world, ghouls are real: undead, zombie-esque creatures that consume dead flesh to survive. Unlike typical zombies, these ghouls are as intelligent as humans; they tend to build lairs near old cemeteries and battlefields, where food is plentiful, but when all else fails they have no trouble making more corpses to consume.
Biologically, a ghoul is an inhuman entity that's taken up residence in a human corpse and uses it as a sort of puppet. Nobody knows where the first ghoul came from, but it's rumored that they have some method of "reproduction": either one ghoul can create another, or new ghouls can arise spontaneously, or maybe both.
The question is: what is this method? It's definitely known that ghoul-ness isn't "contagious" (unlike with some types of vampires and zombies, someone killed by a ghoul doesn't necessarily turn into one). And not everyone who dies comes back as a ghoul. So what is the determining factor?