The Lunar Artifact

When the sirens give everyone Cassie cares about an incurable illness, she must travel across the globe in search of a mythical artifact. Selene's Stone. The stone is possibly the only thing that can cure her loved ones. With only days left before they die, Cassie finally finds it but with terrible consequences. The titaness herself, Selene, sends Cassie on an impossible quest to the least likely of places...the past. Saving those she loves is proving hard enough without her new...
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When the sirens give everyone Cassie cares about an incurable illness, she must travel across the globe in search of a mythical artifact. Selene's Stone. The stone is possibly the only thing that can cure her loved ones. With only days left before they die, Cassie finally finds it but with terrible consequences. The titaness herself, Selene, sends Cassie on an impossible quest to the least likely of places...the past. Saving those she loves is proving hard enough without her new complications. Not only is she falling in love with a merman from another time, she is caught up in the choas of the last Myth War. Worse yet, the side she's trapped on could pit her against Sirena, her father, and her betrothed, David. Desperate not to change time, Cassie must race to save the past and future, or all could be lost...

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The Lunar Artifact is the third book in the Sirena's Children series.