Quote Originally Posted by Anarion View Post
Totally correct, but I think you could do corruption/outside influence without messing up that character arc. Here's how I would do it, though this isn't the only way.

First, set up that the royal pony sisters are especially powerful, and as a result especially open to the influence of magic. This could be something from their earlier lives, or even a scene with Celestia to show that they're similar.

Second, have Luna go through her whole misery at not having anyone appreciate the night, sinking deeper and deeper into depression.

Then, have Luna get possessed while she's in the depths of despair, and give her just one instant to realize what's happening and show her panicked eyes as it consumes her.

That would allow Luna to be aware and feel responsible for everything that happened due to her own depression bringing on the possession, while still having a powerful malevolent force controlling her that could later be banished.
Hmm... Well if it HAD to go in the "evil force possesses her" route by the end, I'd rather the bad thing (I'm just going to call it NMM) was presented as more of a temptress than something that aggressively took control. Egging her on about how much ponies hated her and her night until she finally agrees to accept it of her own free will, even though she clearly knows better and recognizes that NMM doesn't mean well.

That way she's at least an active participant and thus morally culpable. Your way still casts her as a victim.

But I'd still rather she just snapped on her own.