The Fatal Scythe

«The Fatal Scythe» is the boss of the hidden dungeon located beneath the Starting City on the first floor. Its armaments include a large powerful scythe, ability to phase in and out of walls at a tremendous speed. It was fought by Asuna and Kirito when they were trying to help Yulier find her guild's leader Thinker, who had been trapped in the area's safe zone. Its battle level is unknown, but Kirito suspected it to be as powerful as the 90th floors' bosses, proven when it was able to deplete a large number of both Kirito's and Asuna's health with merely one swing of its scythe. It was only through the intervention of Yui that The Fatal Scythe was defeated.

It seems that it is even more powerful than the The Skull Reaper, though it does not inflict as much damage. It is unclear, but one can easily assume that Kirito and Asuna would not have survived the fight, had Yui not intervened by using the GM console.