I wanted to address this post, I'm not sure why, I mean people can use a thing however they like, but I thought I'd just write out my preference.
So, if you approach attempting to answer this question thru system, you abandon story, which is fine, but realize what's taking place. You can make a system that addresses each creatures natural enemy, or trigger to violence. For example trolls hate fire, and will attack the closest thing giving off flame or heat. Orcs hate the smell of dwarf, and will attack them purely on scent. But none of these things are making it any more of a story. They're adding detail, but detail is just one very small part of something, and it can distract from a premise, if you even bother to have something approaching it.
If your desire is to empower your friends, connect them to this thing that's going on, story is a good way to do that.
Maybe this monster lost it's mother to one of the magical weapons, now in the hands of the current adventuring party? Isn't that a juicier motivation? You could even flashback to when the monster was just small, seeing it's mother killed.
If you're just making a system to resolve questions, that has no attempt to show story, it's gonna fall flat.
It's why Pandemic is a shitty rpg. It's the opposite of story. It removes the human drama, through distance and distillation.