Freed (#3 Flamestone Trilogy)

Now with no choice but to go forward, Elaine and her new friend, Weslie, must descend into the terrifying Dweller mines to rescue their friends, who are now trapped in the deepest reaches. That's bad news, because there are monsters down there they'll need to deal with, along with other unspeakable dangers.

But rescuing her friends won't be the only dangerous task waiting for Elaine. There's also talk of a Heart of Flamestone far underground, and it might hold the secret to stopping the Dwellers and freeing all their slaves for good. But worst of all, Elaine's now weighed down with the knowledge that she must also kill her evil stepfather to stop his system of slavery. He's in pursuit and if Elaine can't find the strength to kill him and tear her family apart, he's going to make sure she never sees the surface again.