Ever since Luna was a filly she had this feeling in her that she wasn’t supposed to be a filly but a colt and due to her status and being so young and not fully understanding about this feeling she had surpressed that thought for many moons but as soon as she had married Pinkie she felt those feelings resurface. She had surpressed it for so long that when she finally told Celestia she broke down into tears and Celestia really didn’t know what to think but Luna was afraid of Celestia turning away from her because of it. Celestia had her suspisions of this but being young herself she didn’t really know about this either but after everything that had happened and seeing her sister looking confused and scared broke her heart. They had a long talk but in the end she told her that what ever she wanted to do she would support her and hearing that made Luna feel a lot better about it and after talking with this with the others of course their was shock but they saw this was something Luna had thought about for a long time and knew this was what she wanted.
 
Due to a special spell that had this power to change Luna into a stallion but it took about a day to take effect after the spell was cast she began to feel tired and could barely keep her eyes opened, Celestia helped her sister to the bed, the time she woke up she felt….different, she got up and went to the mirror but instead of seeing the mare she was, she saw a stallion in her place and this was shocking and realization set in, the spell worked and for the first time she felt….complete, happy, HE felt so happy he felt tears fill his eyes.
 
This new look required a new name, he remembered when he was young when they were talking about things before him and Celestia we’re born and they asked if they were born colts what they would have named them, for Celestia they would have named her Solaris and for Luna she would had been named Osiris….Osiris she always liked that name and so when the time came Luna had his name legally changed to fit his new self and for the first time in his life he was truly happy.