NoDictionaries   Text notes for  
... coniuratos erat augebatque scelera, in quorum ultionem consenserat. voci...

A difficult phrase. "in" denotes purpose; "quorum," referring to Nero and his accomplices (Tigellinus et al.) is objective genitive; and "ultio" means "vengeance/retribution," so "for <wreaking> vengeance/retribution upon/against whom."