The Dark House at the Crossing of the Worlds
When Finnegan and Laila explore a hidden room in Laila's aunt and uncle's house, they end up in a terrifying world filled with monster people. Finnegan is saved by his grandmother, but only after she and Laila are left behind in the other world. Chased out of his village, he is raised by a tree-man named Drenlan, who teaches him a bit of magic. He also tells Finnegan a little about the monsters he faced--the Kelim. When Finnegan turns eighteen, he decides to go back to the other world to save his grandmother and friend, or die trying.
Read part of the first draft here.