So, while riot doesn't confirm and explain existence of Underworld in Runeterra, we can be pretty sure that there is one. gatekeeper skin looks like he is guarding the entrance to underworld. underworld skin is basically him returning from underworld. underworld skin (even if he is meant to look like Shadow Isle champion) is demonic spirit version of Wukong.

Yet, the big question remains: what one would have to do, to be damned to go to the underworld?

So, like bible have 7 deadly sins and if you done any of them, you are going to hell, so could be in Runeterra.

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So, these could be 7 deadly sins of Runeterra: Pride (the reason why Wukong was sent into underworld) Deceit (the reason why Twisted Fate was sent into underworld) Rage Necromancy (that is why shadow isles creatures shares look with underworld skins) Greed Worship of the void (well, because void is evil) Madness