Discord's Apple - Carrie Vaughn

When Evie Walker goes home to spend time with her dying father, she discovers that his creaky old house in Hope's Fort, Colorado is not the only legacy she will inherit. Hidden behind the basement door is a secret and magical storeroom, a place where wondrous treasures from myth and legend are kept safe until they are needed again.

Of course, this legacy is not without its costs: there are those who will give anything to find a way in.

With the help of her father, a mysterious stranger named Alex, and some unexpected heroes, Evie must guard the storeroom against ancient and malicious forces and protect both the past and the future even as the present unravels.