Sacred hearts - Steve Niles

For Chris, Dirk and Shelley, life in their small town has always been quiet. But they have all heard about the Franklin Farm a place they should never go near. When the three best friends decide to build a tree fort in the woods, they dare each other to go to there in search of wood, and find themselves in a strange world they never expected, setting off a terrible chain of events.

In this place, monsters are created, and an ancient evil from an old family curse is rising within their little town. But in all the chaos, they come to learn that not all monsters are bad. Some are special some are born with a Sacred Heart

From his horrific beginnings, this special Monster was born from a broken pact with evil, and somehow a door to a dark world had been opened. With the help of their new Monstrous friend, the kids try to stop the destructive forces happening all around them, and end the curse once and for all.