Demons' Crest, Vol. 3 (novel)

The students of Yukihana Elementary School Class 6-1 remain trapped inside Althea, a mixed-reality hellscape. Yuuma wants everyone to work together to escape this nightmare, but unexpected treachery leaves several of his classmates turned to stone. A surprise attack with unclear motives, betrayal from a trusted ally, a disturbing new stat called a "demonization ratio"...and now, Aria has transformed into a demon, just like Sawa! In order to survive, the students must learn to coexist with their demons. But what will this mean, and what price will they have to pay?