Za-Naron is one of the first seven elder Aeons, the very first aeons who founded their race on Solvadol, whose physical forms were disintegrated after 100 millennia and their spirits were each placed into the Soul Star, the most powerful treasure of their people, to exist forever.
When the aeons were redeemed thousands of years later by the Turtles and their friends from the darkness in their souls, they gave April a fragment of the Soul Star as a gift. April has since used the power inherent in the Shard on various occasions to reinforce her own psychic powers when needed throughout Season 4. However, her mind unconsciously came in contact with Za-Naron, whose consciousness lived in the Soul Star fragment, and over time, April fell into a sinister influence that made her addicted to the crystal. It is not clear whether this influence originated from Za-Naron herself, or April's desire to help her friends and fellows at all costs, but Za-Naron's corruption was responsible and her increasing negative influence finally had a boomerang effect on April.
Za-Naron's apparent goal was to rid humanity of its "corruption" by eradicating the "unworthy elements" in its ranks. However, since her control of April was not absolute, she tried to persuade the girl with additional whispering to join her plan, allowing her full control over April. One of the acts she was attempting to implement was the telekinetic disintegration of Donatello; but this act and the pleas of her remaining friends made April rebel against Za-Naron's influence and tear the crystal from around her neck. When it shattered in its fall, Za-Ranon rose from the rubble in a corrupted demon form and tried to take revenge on April. April, however, was able to kill Za-Naron with a blow of her Tantō and subsequently used the last remaining remnant of the Soul Star to fully restore Donatello.