Cool Myth! Simple, but atmospheric. I like the trope and mist is always a plus.
Note: In the Omen, the merchant and guard are described as having swords, but their profile states dagger and axe respectively.
Also, the Omaya has an immunity, but her stats otherwise make her a pushover - and piercing/slashing weapons are super common. Really, only knights with maces or the odd hammer or staff can't harm her, but even those could just draw a dagger. I'm not sure if it goes too much against the spirit of the original folklore, but maybe change it so that she can only be harmed by proper swords? Those aren't quite as common with the Knights, so it might give them a reason to search a smith or whatever for a proper blade if no one rolled a knight that starts with a sword of some kind. Just my two cents though. :)