>>12779
You awoke an old memory in me. I've read that fic, back in 2012. I remember there being... two versions (I started typing this and then looked around FiMFiction).
99% sure I found it. Sunset by Ciroton:
https://www.fimfiction.net/story/19676/sunset
>Tonight would be the night; she could feel it. After about an hour of combing over the unicorn's details, the princess stepped back to admire her work. If it weren't for the clay, a pony could mistake her for being real; just as she hoped. "I do believe you are my greatest creation, little one." Celestia spoke to her clay mare. "Even more impressive then Canterlot itself, I would say."

>The Princess beamed with more pride then when she took control of the sun from her mother those six thousand years ago. If only she could be here today to see her greatest triumph! Not only had she created a masterpiece sculpture, but soon she would create the perfect vessel for her dear Luna's spirit to be poured into. Then, she could kill Nightmare Moon with nothing to hold her back, and finally apologize for not paying enough attention to her beloved sister.

>She almost felt pity for a moment, since she would basically be creating a life for the sole purpose of dying. But then she reminded herself that the mare staring at her would have no destiny and no spirit. Celestia perked up a little bit at this rationalization. Besides, she could be honest with her so that it wouldn't hurt the filly. That would make everything morally right... right? With a shake of her head, the princess shook off the moral dilemma; she had more important things to take care of right now.
From what I skimmed through, Twilight does go to Rarity to help with pest control in Chapter 2, but I didn't see macabre curtains of death in my skimming through of it.It was a Newspaper that seemingly starts her spiral. I know I read this and its sequel For Want of A Dawn way back when. (?related by same guy and diffidently some AU going on here but I don't know how related, I thought Sunset was a mostly stand alone story but this from hazy memory). Thanks for unlocking 2011 to 2012 memories!