Demons' Crest

Sixth-grader Yuuma Ashihara and his twin sister, Sawa, are playing Actual Magic, a brand-new virtual reality MMORPG, when it suddenly begins fusing with the real world. Amid all the chaos and confusion, it becomes clear that death is lurking around every corner, including when Yuuma runs into his classmate Sumika. Before, she was one of the cutest girls in his class, but she's since undergone a terrible metamorphosis—into one of the game's horrifying monsters!

(Source: Yen Press)