Some time after Luna talked with Nightmare moon, some strange news about her spread Equestria. However, Celestia persuaded the ponies that it wasn't Luna's fault. They got persuaded. The news was that Luna destroyed a building in Canterlot, out of her madness. Celestia knew what this meant- it meant Nightmare Moon was returning, & the Nightmare forces were trying to get hold of her sister once more. Knowing this, Celestia despaired. Nightmare Moon was surely going to return.
She knew what was going to happen to herself, so, she wrote to Princess Cadance:
Dearest Cadance,
You should be knowing what happened with Luna today. This means that those Nightmare Forces are trying to control Luna and turn her into Nightmare Moon once more. And very powerful at that. So, if there might be any news about me disappearing, please do take over, for Equestria. You are my only hope left for ruling Equestria. I have to turn only to you because Twilight doesn't really have much expirience ruling a kingdom. You have experience ruling a kingdom.
Yours, Celestia.
Celestia sent it to Cadance and tried finding a way to get Luna's students out of danger. Then, evacuation homes were built upon Celestia's order. It seemed that suddenly Luna became like Nightmare Moon, but few moments later, she was back to Luna. That meant the nightmare forces hadn't fully taken control of Luna.
It so happened that once, during a class she nearly destroyed the place, until Trixie jumped on her, telling her to snap out Of it. However, Luna, or rather Nightmare Moon escaped. She attacked Celestia. The class was called off.
"Sister, why do you do this?" Celestia asked, sadly.
"Well, you should know, Celestia, and I am not your sister!" Luna cried & ran away.
Celestia chased after, but of course, Luna noticed. They galloped up a narrow, winding, rather old fashioned, torch lit stairs. First Luna summoned a locked door, making it appear before Celestia, but Celestia summoned a strong axe, & broke the door down. Next Luna shot a wave of arrows at Celestia, who blocked them using a shield. Finally, they reached the top . It was a dark, cold room, With a window at the top.
Luna said "you will now face your doom & rot away."
Luna rose the moon, then dramatically, her teeth sharpened, her mane expanded, she darkened, armour appeared all over her, and her eyes turned evil. She turned into Nightmare Moon!
Soon news spread over Equestria about Nightmare Moon. Celestia had disappeared. Ponies were put in evacuation homes, and Princess Cadance took over.