"What light reaches below is filtered through fumes leaking from the tangles of corroded pipework and reflected from the stained glass of its industrial architecture." That is Riot's Zaun in one sentence, and the next one is the surprise: "though it exists in perpetual smogged twilight, Zaun thrives, its people vibrant and its culture rich". The undercity is not a slum in Riot's telling. It is a second city, "a dark mirror of the city above", and it has more champions than the city above does.
This is the hub for Zaun: what it is, what chemtech is, who holds power in the dark, what Arcane made canon, and what the undercity would be in the Riot MMO. The game parts are speculation and labelled. The rest is Riot's.
What Zaun is
The district that fills "the deep canyons and valleys threading Piltover", below the cliffs the upper city stands on. Riot's overview says the two "were once united, but are now separate, yet symbiotic societies", and that Piltover's wealth "has allowed Zaun to develop in tandem". Goods that arrive at Piltover's sea-gates find their way down; people who cannot afford the cliffs stay below. The Gray, the smog that the overview describes as filtered light, is the region's weather.
Zaun's relationship to Piltover is the subject of Arcane, and the upper city has its own hub at Piltover.
Chemtech
Piltover industrialised magic; Zaun industrialised chemistry. Chemtech is what the undercity does instead of hextech: compounds, augmentations, engines that run on volatile substances, bodies rebuilt with them. Singed is its worst scientist, and the chemical weapons Noxus used in Ionia were his. Viktor's Glorious Evolution, the replacement of flesh with machine, is its philosophy. Renata Glasc's empire is its business.
For a game, chemtech is the other half of the crafting system that hextech supplies, and the half with a cost: augmentation that changes the body, in a setting that has already drawn what that looks like. We put a chemtech brawler on our speculative class list in Riot MMO classes for that reason.
Who holds it
The chem-barons: the industrialists and crime lords who run the undercity's districts, of whom Renata Glasc is the one on League's roster and Silco, in Arcane, the one everyone knows. Against them, in the series, Ekko's Firelights, the young people of the undercity who fought for it rather than for a baron. Between them, everyone else.
What Arcane fixed
Arcane is canon since October 2023, and it is a Zaun story more than a Piltover one. Vi and Jinx grew up in the undercity; Vander, Silco, Ekko and Viktor are its people; Warwick, in the series, is what Vander became. The look of the Lanes, the bridge, the docks, and the fate of every one of those characters at the end of the second season is now fixed, and any Riot MMO set in Zaun is set after it. What that does to the game's timeline is the question in when is the Riot MMO set?
Who lives there
Riot's roster, fourteen strong: Blitzcrank, Dr. Mundo, Ekko, Janna, Jinx, Renata Glasc, Singed, Twitch, Urgot, Viktor, Warwick, Zac, Ziggs and Zeri.
Arcane's people: Jinx, Ekko, Viktor, Warwick. The scientists: Singed, and Dr. Mundo, who experimented on himself. The powers: Renata Glasc, the chem-baron; Urgot, who came from Noxus and rules the Sump by strength; Viktor's followers. The made things: Blitzcrank, a golem built to clean the undercity that learned to care about it; Zac, a living compound that escaped its makers. The street: Twitch, a rat who drank what the sewers offered; Zeri, whose electricity comes from her own body; Ziggs, a Yordle who left Piltover's academy for Zaun's explosives. And above it all Janna, the wind spirit the undercity prays to, who clears the Gray when she is asked.
Fourteen is more than Piltover's eight, and it is the count that says which city Riot's writers find interesting.
The places
The Lanes, the Sump, the docks, the bridge up to Piltover. Legends of Runeterra painted the two cities as one region with 222 illustrations, and the undercity's cards, the Sunken Temple, the Candlelit Prayer, are the best pictures of the canyons outside the series.
Zaun in the MMO, as a guess
A reading, not a report. Riot has confirmed the setting and nothing about which regions ship, or how.
Zaun is the lower half of the game's likeliest capital, and the half with the content. A vertical city is an MMO's favourite geography: the cliffs of Piltover above, the canyons below, and a staircase between them that is also a class boundary. The chem-barons supply the factions, the Firelights supply the resistance, and Arcane supplies an audience that already knows every street. We argue in starting zones that the twin city is the game's fourth beginning; if that is right, Zaun is where the fourth beginning gets dangerous.
It is also the region where the reset's demand for something "different enough" has the most room. Nothing in the genre looks like the Lanes.
What we do not know
Whether Zaun and Piltover are one zone, two, or one with a border the player can feel.
Whether the chem-barons are a faction the player can join.
Whether Riot lets a game show the undercity's augmentations on a player's own body, which is the point of chemtech and the hardest thing to build.
For Zaun among the other twelve regions, with the numbers, see the 13 regions of Runeterra.