I'm looking for a story where Celestia declare Discord a traitor after Tirek kills Twilight and her friends.

Wish I could help you...

It does sound familiar...

Do you know if Tirek or a minor character is tagged or not?

If so you're best bet is probably to go through their back catalog.

Tirek's back catalog has grown quite a bit since people started writing about him and Cozy Glow, but it's still probably your best bet if you can't remember anything more specific.

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I mean it's taken place in season 4 Twilight's Kingdom alternate ending