There was a story where Twilight was working with some unicorn who was aggressive and a pyromaniac, and IIRC killed Luna through a teleportation spell. Anyone remember the story?

Yes, I remember. Researcher Twilight by NATOstrike

It's been removed
But it's on fim fetch
https://fimfetch.net/story/49113/researcher-twilight

7503325
What, seriously? I remember there'd been a new chapter posted a while back so I was hopeful that the author would start working on it again, but I suppose that's how they get you. Man that sucks.

7503330
Yeah I really liked it
I don't know why it got removed

I remember there being either blogs or comments on the author’s user page but they’ve scrubbed that entirely. Was online just yesterday, though. Fucking cancer wasn’t enough to kill him nor his story and now he just ups and deletes everything? Lolwat.