A Human Problem

Cassin leads a simple life. Someone has a problem with a monster. Cassin solves it. Far from easy work, but it pays better than the alternative.
A letter addressed to his name arrives while on a job, strange enough for a person in his field but for him, more so. With only a map, a request, and the promise of riches, Cassin's next job is decided.
Not long after his arrival, he learns that things in this isolated village are not as simple as he's used to. With events only becoming more bizarre, Cassin is brought eye to eye with his own morals. Or the lack thereof.
What is a monster? Who gets to decide? And does it matter if you're getting paid?