That's not my personal headcanon, but I have to admit that the idea is interesting, thus, this pic came to be.
EDIT: I've read the comics and Sombra has absolutely nothing to do with Luna's transformation. oh well.
Luna & Sombra © Hasbro
: Luna oh my
: me hey guys you uhmingiling or something like that ya? Okay then
This piece of art is really beautiful!!! Love it!!!
Just kidding. This is absolutely brilliant, great work!
Luna: "Th-that, no, they..."
Sombra: "My point, exactly."
The emotions captured in their expressions is amazing! Sombra's expression makes me think of pity and condescension: he's as much manipulating her as he is trying to help a fellow ostracized pony of darkness, trying to get past all the near-brainwashing of Celestia ("Give them time, they will come around, dearest sister."). Luna on the other hand is trying to remain faithful/loyal, but she's unable to form a proper argument against Sombra's and she knows it.