At the end of Alicization, Kirito and Asuna are forced to remain in the Underworld for over 200 years, and in this time, Kirito has gained immense experience and knowledge in general, especially as an engineer, ruler, politician, and swordsman (Though this experience and knowledge is erased from EoA Kirito upon getting out of the Underworld). In his early days as ruler, he was able to reunite and reforge all of Caldina back to its former glory, reinstating planetary peace. After finishing up the last of his home planet, Kirito turned his attention to the vastness of Outer Space and developed interstellar-capable starships, colonized other planets, set up star ports, space stations, space military divisions, and an interstellar travel route, becoming the cosmic emperor of the (now interstellar) Human Empire, gaining the title "Star King".