Sylas's Introduction has had more lore impact than any previous champion: Change my mind.
Sylas has been a literal wrecking ball when it comes to Demacian lore and the events within Demacia itself. While it has been spread across a few months since his release, Sylas has:
- Made Lux come clean that she has magical ability to her brother and family.
- Either killed J3 or (more likely) was the crux to force another party to kill J3
- Has had J4 assume the throne so he is no longer prince.
- Plunged Demacia into Martial Law/Civil War.
- Mageseekers. Just mageseekers dude. You can't trust them.
- Has given us a look into Xin Zhao's aftermath and relationship with J3.
- Introduced us to the extended (and possibly very Lawful Neutral/Evil) Crownguard family with Tianna and her husband.
By all accounts I can't think of a single champion who has spurred this much change and character growth in a region. The only event that can possibly come close is Gangplank's rework but even then, the scale of the changes was on a much smaller scale as Bilgewater is a "smaller" region overall. Not to mention the story in Burning Tides was a little shorter and smaller than Sylas.
I'm usually critical of Riot's inability to follow-up stories but they've certainly made Demacia as a nation progress since Sylas's introduction.
EDIT: Azir is brought up a lot but I disagree with Azir. Azir's introduction wasn't a story progression of Shurima but rather resetting the story of it/retconning what occurred. A few characters did get progression (Mostly Sivir and Cass) but most of this was just setting up Shurima to be an actual nation rather than a loose collection of psuedo-egyptian ideas. Conversely, Sylas doesn't retcon or change any history but rather takes the story of the region and makes it continue in a meaningful and interesting way.
Azir is a retcon and rewrite. Sylas is Chapter 2.
and
since Kayn also introduced Rhaast and we got more insight to the darkin and a retcon for Varus' story.
took over noxus and ended the war with ionia sorta. It was started by the previous dude iirc. But he changed the way the country governs largely.
pretty much runs the lands noxus is build on from the shadows, and has been doing that shit since morde.
"I sold some bottles of chemicals once."(Ionians mad x24)
"i got this nice as heck stone collection, world might blow up if someone finds it tho"