When the cup was broken into seven pieces, Anubis made an agreement with Amneris that once every 25 years, on a special date, at a special time, a member of Amneris' bloodline would be able to reconstruct the Cup of Ankh. This person would be born during a certain month, on a certain day, and a certain time, which all fall on the same number (example: Sarah Frobisher-Smythe who was born during the first month on the first day during the first hour). It was originally believed that the Chosen One was Joy but in the finale it was revealed that Joy was born on the right day, but at the wrong hour (the 19th hour - which is also known as 7pm.).
The true Chosen One is Nina. It is shown that the Chosen One might have powers relating to seeing ghosts and spirits. The Chosen One also has an opposite who is called the Osirian (Eddie, for Nina). The Chosen One is also probably very loyal and of pure heart as both Sarah and Nina would do anything for their friends and Nina could enter the Egyptian afterlife while Rufus could not. The Chosen One and Osirian can't be together because 'bad things can happen'.
All the way through the first season, The Secret Society believed that Joy was the Chosen One because she was born on July 7th, at 7:00 p.m (which they thought was the seventh hour but is truly the 19th hour). She was actually kidnapped for most of the season because they were scared that The Betrayer (Rufus) would find her. Later in the series Sibuna believed this claim as the truth. During the ceremony Joy couldn't assemble the cup and gave up. So did the Secret Society. Nina, being born on July 7th at 7 a.m. made her the Chosen One. Then Nina and the others arrived and Nina assembled the Cup of Ankh.
Nina, was not known to be the Chosen One until the end of Season 1. She was the Chosen One because she was born on the true seventh hour, 7am, not 7pm, on the seventh day of the seventh month. With the help of the Sibunas and Sarah, Nina successfully assembled the Cup of Ankh. The Chosen One is protected by her Osirian, who also hears voices like the Chosen One. In Nina's case, her Osirian is Eddie.
