Sword Art Online Progressive, Vol. 2 (manga) — Reki Kawahara

The party is shaken when their leader, Diavel, is struck down by the boss of the very first floor. Asuna and her teammates are confronted for the first time with the cruel reality of their situation: Sword Art Online is no mere game--death here is final. Their commander lost, the remaining fighters begin to lose heart and plan their retreat. But the fight must go on if they are to escape this game world, and in his dying moments, Diavel managed to convey his appointment for the next commander--Kirito!