Unanswered//butterfly: Sword Art Online

Story

The world’s first full-dive VRMMO, Sword Art Online—known as SAO. What was meant to be a dream come true has instead transformed into a deadly game where dying in the virtual world means dying in reality. Emirun and Rex, who once knew each other in the real world, reunite by chance within SAO. They join a scouting party to learn how to survive in this unforgiving world. One day, however, they encounter a mysterious PK (Player Killer) known only as the Black Swordsman. When Emirun rushes to the scene, she finds Rex collapsed in despair, and the Black Swordsman standing over him.

Synopsis via AniList (English).