Help from Unknown Goddess

What if Rhea and Kronos had another baby what if she disappear after her heart was broking by the one she thought be her love. What if she was watching her nephews and nieces aiding them. What if Percy and his cousins tell the Gods that someone powerful as "The Big 3" was watching him How will the Gods reacted, how will demigods reacted? (Take place during Last Olympian)