Lastly, I think it would have been plausible that said Nightmare Forces harassed Princess Celestia during those 1,000 years that Luna was gone, feeding off the despair and regret she kept buried over entrapping her sister in the moon. Certainly they can have more than one host at a time, and one being incapacitated was very inconvenient for them.
Celestia would have to constantly fight against their influence and keep her head held high for her kingdom, all alone.
Idk just some weird thoughts I’ve had for a while.”
Andy Vs The Nightmare Forces trying To make Celestia their New host.
It looks like The master of darkness decided to go to Equestria instead of Andy.