Noxus is burning in Swain's teaser.
If you look closely at 00:16ish you'll see fires in the city. Some look like bonfires on the streets, some might be burning buildings.
This is what I think is happening.
Swain's ascension to Grand General story will be the focus of the narrative part of his rework. He will either take the throne when the city is under threat or cause the riots himself. This is most likely the anti-Darkwill rebellion.
"But the Grand-General Jericho Swain knows there are many kinds of strength—and the greatest leaders of the next age will be those who can wield them all..." - he will unite several vastly different, maybe conflicting forces, to achieve his ambitions (is this where the potential changes in Sion's promo become evident? That last one is tinfoil-y.)
EDIT: Here's a picture with the fires and all:
