Swain foreshadowed Sylas.

I wouldn't really consider that foreshadowing Sylas, to be honest. Granted, Sylas did have the ability to see magic, but, that's about all there is in terms of similarities to Swain's quote.

I think it's more likely that whatever calamity Swain foresees and is preparing for may start at Demacia - perhaps due to the World Runes that Ryze is hiding in a vault below a forest of magic-absorbing trees? Maybe there's something else entirely about Demacia - we really don't know, and I'm personally doubtful that that quote refers to Sylas.

I feel like that's more of a reference to Ryze's World Rune vault. or perhaps the need to have petricite as a counter to Void energy/Black Mist/Swimming City stuff. Proooobably the former, though.

Swain also hints at the fact that petricite absorbs magic not nullify it. When he gets a soul fragment from he says "Ignorant of the very magic you carry into battle. How Demacian."

Huh, I have a theory: Sylas is extremely Radical. He’s willing to go very far and do some pretty terrible things. How much do you bet if he learned of the World Runes he’d use them? Their inside Demacia in a PETRICITE forest, something he already knows very well. He would likely sense the world runes if he only got close enough to them, something I doubt anyone not as rehearsed in Petricite knowledge could do.

I don't think that's what was foreshadowing Sylas. If anything, his quote to Galio is what more or less foreshadowed Sylas, as it hinted at Durand having secrets. "His secrets died with his sculptor."