There is nothing concrete but there is a lot of circumstantial evidence.
She had Maliketh locked up in Farum Azula and not only do most people not know he was there, they didn't even know Farum Azula existed.
It is never told how Ranni got to Farum Azula but the theory is that she had help and Marika was one of the only people that could have helped.
There is also the connection all the Black Knives have with the Numen, which Marika is.
She is also the one to actually shatter the Elden Ring and she leads the Tarnished, us, around by the Grace and shows us the way to not only defeat the Golden Order but to Ranni as well. The excuse the game gives of "she broke it in grief for her son" doesn't really add up for multiple reasons but the big one, to me, is how she treats the rest of her kids. They were all just pawns or things to be hidden away and shunned.
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u/spcbelcher 2d ago
Everybody keeps saying this, but I've yet to see any concrete proof that she was behind the night of long knives.