The Underworld is the mysterious virtual world in which the Alicization Arc takes place. The Human World is where the Humans live in the Underworld, it’s a perfect circle with a diameter that measures arround 1500 kilometers.
It’s divided into four empires: Norlangarth North Empire, Eastabarieth Empire, Wesdarath Empire, and Southacroith Empire.Iit’s capital is Centoria, from where it’s ruled by the Axiom Church and the four empires.
In the Alicization Arc, Kirito has been assisting in the development of the fourth generation full dive interface, Soul Translator, by testing its thought acceleration functions. However, he has no memory of what he has been doing as a result of his memories being suppressed, voluntarily, by the system. He is attacked and poisoned by Johnny Black (The third culprit who escapes) in the real world and wakes up in Underworld with no memory of the game or how he got there. He meets up with Eugeo and helps him out of some tough situations, during which he partially remembers his time with Eugeo and Alice during his “childhood” in Underworld. He then decides to aid Eugeo in finding Alice while he searches for a way out, back to reality.