Sword Art Online 8: Early and Late (light novel)

The saga of Sword Art Online continues with three side stories set during the Aincrad and Alfheim eras. The game of death took its toll on every player in a different way--when one turns up dead on the fifty-seventh floor, the game's afoot, and it's up to Asuna and Kirito to crack the case. There's also the story of how Kirito got his Excalibur sword in Alfheim (with a little help from his friends).

And when the game of death first began, Kirito learned a difficult lesson--one he would never forget.