Sword Art Online, Vol. 20 (light novel)

The Taboo Index and the Law of Power have both been breached by a single murder. The shadowy culprit's intentions are clear as day: the human realm and the dark territory must go to war again. With the help of Iskahn, commander of the Dark Army, Kirito seeks to bring the criminal to justice. But as they conduct an investigation with Asuna's help, disaster strikes again! Ronie and Tiese have been poisoned!