What is sword art online 15: alicization invading?

Sword Art Online 15: Alicization Invading is the fifteenth light novel in the Sword Art Online series written by Reki Kawahara. It is the second volume in the Alicization Arc, following Sword Art Online 14: Alicization Uniting. The story follows Kirito as he explores the Underworld, a virtual world that he was sent to during an experiment by the Ministry of Defense. Kirito must navigate through the various factions and groups in the Underworld and protect his loved ones from danger. The novel was published by ASCII Media Works on August 9, 2014, in Japan and later translated into English in 2019.