I would be very, very impressed if this is what they were doing, and I’d be happy for more background on this guy. Maybe he could be an arc villain for the whole season and be finally defeated in the series finale?

Yes, I’m for saying it’s most likely an unrelated artifact. The colour scheme (red, black and steel) are common “Evil” colours, Sombra doesn’t have wings, Sombra doesn’t control ponies by any means than physical force (remember, the crystal ponies were in chains), and - most of all - FiM has never directly referenced previous episodes.
 
Plus, you have other stuff, like how Trixie was never actually confirmed to have that medallion, and that the staff may be intentionally making it similar to Sombra to encourage fantheories and fics. I mean, they’re completely contradictory points to each other, but it still ups the chance that Sombra and Trixie are completely unrelated.

Uh, they are not afraid to have death on the show, did you already forget the funeral?

I’d rather have him remain dead.  
i know he was really undeveloped but I just want to be sure that they’re not afraid to have death on this show.

Oh, and in before someone makes rule 34 of such a scenario.

Inb4 Trixie finds the horn and in her depression uses it as a dildo.  
It then enters her body, and finally rests in her womb.  
And well…

Yes! The fanon isn’t necessarily ruined!

That necklace mught be an Element of Disharmony…  
Might be a good excuse to expand the saga and earn more profits..

@RandomFanguy: That’s what I said to myself.
 
60 images ago.

Ok, starting now. No more spoilers for me.

I will put my entire thoughts on these theories in one gif
 

@Arne  
i afraid that the necklace and the fact that she defeated Twilight cancel that scenario (but she still could be a bitch).

Interesting theory there… I think I’ll share mine now.
 
Trixie will be evil because she a total bitch.

Actually come to mention it… there’s a lot more of Dark Magic seeming to come out of the woodwork.
 
Trixie using an artifact that’s beyond her control with full knowledge of what she’s doing and it turning against her.
 
I’d like that.

I sure hope so!
 
Woo! Sombra!

Just want to throw this out there, but twilight was wearing a necklace in the preview when she de-ages her friends. It’s a split-second shot, just keep your eyes at her neck. Watch for the green gem.

OH yeah!! that mean more sombra!!

Interesting theory, but I still think that outside of color scheme her necklace bears no resemblance to King Sombra. I think she just stole it from some powerful wizard she had accidently came across, and it’s modelled after his insignia or something.

YESSSSSSSSSSSSSS I WANT MORE SOMBRA!!

Yeah, seems plausible.