Fantasy: Evil sealed in isolated valley and MC, shepherd man, goes where he shouldn't and releases it

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I read this book between 2003 and 2008. It was in the regular fantasy section, and my impression is it was written before there was a real YA thing, so I think it was written for adults. It seemed a little older when I read it. I've been looking in the range of the 70s to 90s but of course I could be wrong. I haven't seen the book for over a decade. It was a paper book.

There is this idyllic village isolated from the rest of the world. Somewhere in this valley is this place where some ancient evil was sealed, and all the villagers are told not to go near it. Obviously our hero is going to go near it at some point, and it starts to stir. If I'm challenged on the details, I might backpedal. (Am I SURE that it happened in a valley? Am I SURE that the MC was a shepherd? I think so but I wouldn't put real money on it.) More than that, I'm afraid I don't remember. Any help is appreciated, the library where I read this is gone and there are no records of my history there.