Some monster weaknesses are well explained in various works.
Werewolves are burnt by the touch of silver. Probably because silver was often associated with moon.
Vampires can have this weakness too, or it could just be that they feel pain while touching it, or their wounds don't heal.
In some fantasy settings you can only kill an angel or a demigod with a golden weapon. But I have no idea why. Maybe because its used in temples or because it represents one of the capital sins - Greed. Heh... ironic.
And then there is the fext, a revenant from a 30-years war. impervious to all damage, except for glass bullets (or other odd items).
So how can I explain this weakness? (reasonably, without high doses of handwavium.) Real world mythology is as blurry as always and didn't care to provide an answer. Mythology stackexchange also did not provide an answer... yet.
I realize this could have so many different answers, so I will ask just in general
- Why should a revenant/demon have a weakness against glass, or items not intended as weapons?
More about the fext:
It is a revenant, that came back, because he commited a bad deed and now has to redeem himself, or he has some unfinished business on this world, for example a revenge.
His skin is cold as ice and hard as rock. bullets fired at him either miss every time, or deflect off his skin, or are crushed on impact and then the fext throws it back in front of the shooter, while revealing his true nature. Sometimes a direct hit sends him to the ground, only for him to stand up again with the bullet in his hands. Sometimes depicted as cold and emotion less sometimes staring into their eyes tells you exactly what emotion they feel, sometimes this trick starts to freeze you from the inside. After the fext is put to rest, his body does not decompose, his skin will shrink and his muscles will dry or turn to dust. if a situation comes when he has to rise again, he will turn back to his original form.
This creature is partially created from the original slavic myth about revenants that are called "zmrzlík" - Cold one. Warriors that were called back from the dead, to help win the war. Cold to the touch and cold in their hearts, they were great tacticians. Their names later became interchangeable with fext.