New Star Wars series The Acolyte

TheRealJRad said
Well, we don't know that this is a "Sith", to be fair
We also know the Sith being extinct was BS when he said it. A lot of this content from pre-New Hope is specifically telling us just how much hubris the Jedi had and how it led to their downfall. A Sith Lord existing literally under their noses being a prime example.

Edit: I'm not necessarily saying you're wrong, but I don't think this is firmly breaking canon

I'm trying not to think about it to hard, but red lightsaber means sith. I'm sure there a few exceptions, but that is the rule. You assume the current Jedi Order at least learns that.

The red saber is made via bleeding the crystal, and that is a very dark side thing.

https://starwars.fandom.com/wiki/Bleeding
Of course this will make sense, if Mr. Grins kills all the Jedi.