Sword Art Online: Project Alicization Vol. 1 (manga)

"This might be a game, but it's not something you play." - Akihiko Kayaba, Sword Art Online programmer

With no memory of how he got there, Kirito wakes up in a mysterious virtual world, and it's up to him to find a way back home. But with no way to log out and no idea what kind of land he's found himself in, he's going to need allies! As a new tale unfolds, Kirito's journey through the Underworld begins!