Basically the Anon a Miss account is hacked and a video is added, in the video, the face of a figure wearing a mask can be seen, the figure sends a warning to the account in the same vein as the Shido Calling Card from Persona 5, she also disguises her voice

Anon-a-Miss, you have framed an innocent girl and ruined her attempts to atone, you will stop your reign of terror and I know who you are, I have a list of every student who have sent in secrets to you and then had the gall to blame Sunset for what they did, everyday I will expose one of them for the two faced hypocrite they truly are and will strike when they least suspect it, you have until I finish the list to reveal yourselves. Choose wisely

the video is then deleted from the account and the Anon-a-Miss image with Sunset’s Silhouette is replaced with the figure’s, it’s now purple and has the words “Mare-Do-Well”

The mysterious figure then follows up on their threat

Sounds good, I like to suggest instead of Sunset trying to end her life or running away from home.
Let say she found sitting alone crying utterly heartbroken and ever so depressed and suffering alone.

That would do it.

Hm a vigilante against anon-a-miss. I like this.

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Looks promising.
If its ok, may I give a little suggestion?
It would be interesting if the face behind Mare Do Well was revealed at the end and it was someone surprising like Vice Principal Luna, Snips and Snails (they probably learnt a few tricks from Sunset), Fluttershy or maybe even Equestrian Discord (you know, reformed villain helps fellow reformed villain) or someone along those lines

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This is for someone else to use, I was gonna have her be Derpy

Sounds pretty good as well as pretty original I think. Would love to see where this would go.

Also she strikes when people least suspect it and she posts them on the Anon-A-Miss account, despite the CMC's best efforts to keep her out and she does post a video saying she's not Sunset when one of the Rainbooms suspect she is and even sends an message to them

Leaping to conclusions again, Rainbitches?, how pathetic

do it