Percy's Twin Sister

Prologue

The woman started running across the golden, hot sand; heading towards the water. She ran past people who didn't stop to look twice at the lady. She ran north, towards the water and put her hands on the edge of the water's surface. She muttered something and then stood up.
She wore a thin, silver-colored tunic. She had auburn hair and silver eyes. If you looked closely, you could see the air shimmering slightly around her, cloaking her. She was breathing heavily and was looking up at the sky frequently, as if she were scared of something in the sky.
The ocean waves that were once calm, began to churn and grow as a man came up from the water itself. He was literally apart of the water. His upper half was normal, while his lower half was intertwined with the water. He wore gray armor on his chest; had short, black hair; had sea green-blue eyes; and was dripping wet.
He looked at the woman and frowned. "Artemis," he spoke. "What are you doing here?" The woman, Artemis, came towards him and said, "Poseidon, dear friend. I need your help, but we have to speak in private or Zeus might overhear."
Poseidon rose to his full height, legs and all, and walked onto land. "I know a safe place where can be alone and no one can see nor hear us." he said. Artemis sighed. "Thank you for this." He looked at her with an expression of doubt. "I hope whatever you need to say to me is worth it."
She nodded grimly. Yes, yes it was.
Poseidon willed the ocean to take them to a far away island, shielded from mortals and gods alike. They landed on the marble-white sand and Artemis sat down.
Poseidon followed her lead. All was quiet. Then she spoke. "Remember my oath to Zeus to become a maiden forever?" He nodded. "I seem to recall the little girl sitting on Zeus' lap and stating she wanted to be young forever and never marry."
The water washed over their feet as she smiled faintly. "A few millennia had to have passed until I realized that that was a mistake." Poseidon quickly looked over at her. "A mistake?"
She laughed humorlessly. "I love my Hunters, but I would love a daughter of my own. An heir." Poseidon looked confused. "And this involves me how?" She looked uncomfortable. "Athena is a maiden like I am. Yet she has children as well. I want a daughter, but don't want to lose my maiden status."
Poseidon still looked confused. Artemis looked down. "The moon controls the tide, the saying goes." Now he looked uncomfortable. "Artemis, if Zeus...He is my brother...I don't-"
She interrupted him. "Not make a child with me intimately! We could find a surrogate. You meet a mortal, do whatever you do with a mortal, we create a fetus with moon and ocean power, and bless the mortal woman to carry it. Simple."
Poseidon looked stricken. "What is the gods found out? We could be exiled! And poor Sally would find out the child won't be hers!" Artemis spoke calmly. "Then swear to the River Styx that our secret won't be told out of your mouth."
Poseidon looked at Artemis grimly. "You know how serious the oath is." She nodded. "This secret is serious. More than you can know."
Poseidon looks down and solemnly and clearly says," I swear to the River Styx that I will never say a word to anyone about your -our- secret to anyone. Until the child comes of age."
Artemis looks up, outraged as thunder booms in the distance. "When the child comes of age?!" she says angrily.
Poseidon shrugs. "The oath is done. It doesn't matter now." She looks out towards the setting sun. "Get ready" is all she says.