The Castle (Guardian Angel #0.5)

Immortals tend to think they know a little more...make that a lot more...than anyone else. Married to one of the immortal High Council members, Heather knows more than most it's hard to tell immortals they have a bad idea. She'd already tried and failed to stop the High Council's locket collection idea. Now, on the eve of the Grand Opening of the Guardians' new school, she and her fellow time traveling Guardians search for the children who, without a locket to guide them back, find themselves lost when they accidentally teleport into the world. Could the day get any worse?

It does. A betrayal and a sinister plan involving the lockets unfolds to fracture her world. Heather, with a sole surviving locket, suddenly holds the fate of the Guardians, and even the life of her immortal husband, in her hands. Under constant watch by a power-hungry wizard who has decided to create a new Order of Guardians at the Castle, it is a dangerous game she must play to outsmart him. But with each choice more impossible than the last, and promises made that can't be kept, it seems the only choice left is to make her escape.

Starting over, she fears, is asking too much while a past waits unfinished. A world of magic can't be left behind and eventually Heather must make the hardest choice of all.