The Gatekeeper's Curse

A complete adult urban fantasy trilogy. 

As the twin sister of the Gatekeeper between the Summer Court and the human world, Ilsa has mastered the art of hiding amongst the normal humans to avoid the faeries' attention. But when she discovers volatile magic inside a cursed family heirloom, she must learn to wield it before her family faces a fate worse than death. 
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The Gatekeeper's Curse series takes place one year after the Changeling Chronicles. To avoid spoilers, I recommend reading Ivy's story first.