In FE:CSR, Starlight will be quite different than it was in the games. For one thing, it is a plot-related weapon that is used and/or mentioned in every tale.
In the Akaneia games, Starlight was (if I remember correctly) the only tome that could negate the effects of Imhullu (I think Falchion could do that as well, but I don’t remember). In my publication, it is the only weapon that is effective against Black Magic.
Now remember, in FE:CSR, Black Magic exists outside of the Magic triangle/Trinity of Magic (basically, they are the magic equivalent of bows). Unlike bows, however, most higher-level black tomes render the user impervious to all non-holy weapons, with a few tomes impervious to ALL weapons (including Imhullu and Ereshkigal, which I changed to be Black Magic tomes). The only trump card is Starlight, and only two characters can use it. In-game this would be incredibly unfair, but remember, FE:CSR is a FanFiction publication, not a video game. There are exceedingly few characters who are capable of using black magic, and even fewer who are adepts. In addition, black magic corrupts the soul of the user and drives him/her insane… unless the user is an Archsage or has Avram, Loptyr, Medeus or Grima Holy Blood. When they completely loose their sanity, they become puppets of the Devil and typically are sucked into hell, only resurfacing when called upon by a demon existing on the mortal plane and during the End Days, which I will expand upon in Tales 12, 13, 19 and 20. When I write Book 1 (the prequel to the current publication), which contains Tales 1 through 9Ax, I probably will reference the End Days, but by then I will have fully explained them.
IF FE:CSR were a game, Starlight’s stats would be:
Effects: Effective against Black Magic; negates enemy’s skills and weapon effects
*Preference: Gotoh, Morva
Holy Weapons can do damage against most black magic users, but are not necessarily ‘effective’ against them.
If you have any questions, leave them in the comments.