What happened to the "Magma Chamber" gamemode?

That's been a dead meme for years.

That's basically SR midlane with another theme ?..

it turned into aram. although i would love to see it back.. even it its only rotating

Back in 2010, Riot promised that they would create a new map called "Magma Chamber." That initial design, once Riot had done their internal testing, eventually became "Dominion."

However, the meme just would not die. So, at an All-Star event in Shanghai, Riot introduced the 2v2 map in the video and just called it "Magma Chamber." Riot quickly abandoned the map in favor of Howling Abyss for their 1v1 tournaments.

(This is one of the main reasons why Riot never makes promises unless they're 99.999% done with their processes.)

Lord, Magma Chamber. This was first teased waaaaay long ago, like Season 1 maybe even before, as a map bigger than SR that would have longer lanes and possibly support larger teams. Time passed, people questioned where this map was for ages until finally Riot announces that its cancelled. (Later, Tyrndamere would mention that the concept eventually became Dominion and the Crystal Scar.) Cut to years later at All Stars (I think 2013? Sometime around there), and they present this map as a "1v1/2v2" map, designed for this more so than Howling Abyss, which is more about the wacky fun and randomness. Magma Chamber was meant to be the ultimate "1v1 me bro" map and they showed off the mode at All Stars. But the map was never released to live clients, possibly because Howling Abyss already existed, possibly because they felt 1v1s weren't engaging enough, maybe they wanted their maps teams to focus on existing maps (hah, RIP Crystal Scar), I don't know. I don't remember the reason.