I replaced the i with an y because it is a reference to the Greek goddess/personification of night. I did it mainly for symbolism and also because I feel it makes the name seem more feminine than Phoenix because Nyx (the deity) was normally portrayed as female. Phoenyx cross dresses often because she has numerous scars covering her body. For where she lives, it's easier to be a scarred boy than a girl. In "my future" Phoenix/Phoenyx is a predominately male name so she goes mostly by Nyx when not cross dressing.
Beautiful. Not just for aesthetics! Yay! Joy has been brought!