Demons' Crest, Vol. 2 (light novel) — Reki Kawahara

The latest VRMMORPG, Actual Magic, has sealed an entire class of sixth graders in a terrifying game of life or death. Countless dangers await eleven-year-old Yuuma and his friends. One of their classmates has been turned into a monster, and gargantuan beasts lurk around every corner. Their only hope? The demon Valac who has possessed Yuuma's twin sister. Following Valac's guidance, Yuuma and his party set out to save their missing friend, Nagi. But an unexpected encounter awaits them...