The Hidden Secrets of Olympus: The Broken Vow of Artemis (Book 1)

Artemis broke her vow. The vow to stay maiden forever. When Zeus finds out about Artemis' twins, he shuts the doors of Olympus yet again. This time, not only for the demigods but also the goddess herself. Her Hunters no longer favour her. Searching for help and protection of her children, she goes to the demigods for their aid. Thus, they make a truce, a compromise. If Artemis can give them back their presumably dead friend Leo, they shall make place for her and the children.
However, the monsters see their advantage in the situation. In the midst of a confused environment, they decide to attack. This time, they are stronger than ever. Will the combined forces of demigods win again, or shall they be defeated by the powerful mixture of the Roman-Greek monsters? The oracle makes it clear that Leo holds the key to both the problems, but will he be found?

(P.S- This book can be understood only by those who have read all Percy Jackson and Heroes of Olympus book series.)