I feel bad for picturing how that would happen a number of ways, for a fair few scenarios. Most clear cut one that pops into my mind that would throw a fair bit of game lore, far as I understand it, is two monster hunters, long before they were a unified faction against said monsters, from rivaling clans encounter each other in the field and are mid match when the monster itself pops up and has snuck close for neither to fully realize it. Is a hell of a scenario and is probably stupid. But is interesting enough to keep my mind flowing.
You could also make it as a monster hunter who was tasked with killing a fellow hunter who sympathized and protected a monster, betraying the faction and what they stand for.
