The Vampire Diaries: The Return Midnight by L. J. Smith

With the help of charming and devious Damon, Elena rescued her vampire love, Stefan, from the depths of the Dark Dimension. But neither brother returned unscathed. Stefan is weak from his long imprisonment and needs more blood than Elena alone can give him, while a strange magic has turned Damon into a human. Savage and desperate, Damon will do anything to become a vampire again -- even travel back to hell. But what will happen when he accidentally takes Bonnie with him?

Stefan and Elena hurry to rescue their innocent friend from the Dark Dimension, leaving Matt and Meredith to save their hometown from the dangerous spirits that have taken hold of Fell's Church. One by one, children are succumbing to demonic designs. But Matt and Meredith soon discover that the source of the evil is darker -- and closer -- than they ever could have imagined....