You can blame the Commenters for this joke chapter. Note is not canon to story just fun.

Can anyone guess the reference in the Chapter Title?

Nightmare Moon sat on her bed, thankful that the Pink menace was secured. Tomorrow evening the conquest of Equestria begins!

<Moonie, you have forced me to do this. You forced me to bring out the big gun. Pinkie, you ready?>

<Rightie oh!> 'Oh no she can be free, much less in my head!?'

<Okay Pinkie, follow my lead.> Selene began to sing.

<This is the song that doesn't end, it goes on and on my friend.

Some people started singing it, not knowing what is was

and they continue singing it forever just because.

It is the song that doesn't end..> At this point the Pink Menace joined in and the two of them continued repeating the lyrics. Without thinking Nightmare tried to block her ears but, of course, it did not help, as the song was being sung in her head.

<And it goes on and on my friend...> Nightmare began trying various spells, any spell that might shut them up.

<It is the song that doesn't end....>

They. Didn't. Work.

<Some people, started singing it not knowing what it was...> Nightmare's eye twitched, and her mane was frazzled. Everything she tried failed and they kept singing! She had but one recourse left. She began charging her magic...

<And they'll continue singing it forever just because..>
Once all of her magic was charged up, Nightmare released.

<It is the song that doesn't end....>

From Canterlot everypony saw an enormous mushroom cloud destroy a mountain and some thought they heard somepony say.

“Finally it stopped.....”

* * * * * *

“And that's how me and Selenie saved Equestria from the second return of Black Snooty.” The elderly pink mare finished her story to a trio of fillies. Said fillies sat there wide eyed.

“But Granny Pinkie, how did you get free?” One filly asked.

“Ah the young and their innocence.” The fillies look behind them and saw Princess Luna.

“PRINCESS LUNA!”

“Hello fillies.” The trio hugged the alicorn and she nuzzled them back.

“What are you doing here princess?” One filly asked.

“Getting ready for Nightmare Night and visiting my friend Twilight Sparkle.” Luna enjoyed spending time with fillies and adored Nightmare Night despite it's name. However, for the past few decades it has been different for her.

<You know after all these years I still haven't figured out how she not only got out of there before Nightmare blew but got me stuck in your head?> Selene commented.

'I thought you knew better then to question Pinkie?'

<Why do you think I haven't spent the time to ask her.> Luna and Selene's relationship was a strange one. They share the same body now and can both control it but Selene allows Luna to have control most of the time but there are times where Luna is just not in the mood and allows Selene to take over. This relationship has been very entertaining at times. In particular Luna allows Selene to have control on Nightmare Night since they are usually in Nightmare form and Luna still enjoys the night even if she takes a 'back seat' as Selene calls it. While Twilight's last attempt failed to get Selene a new body neither mare is in a hurry. For now Luna never needs to fear being alone again.