Isto 'Vadum is a relative to Rtas 'Vadum, although specifics are unknown. He is known for his brutality in combat, and was widely suspected to be a part of the faction that did not support the Sangheili's truce with humanity.
Isto arranged for hundreds of Sangheili newborns to be kidnapped and taken to Earth to create further tension between the two races in a stealth corvette. He was found by Autel 'Vadam and Fira 'Demal but managed to get away after injuring Fira.
Isto arrived stealthily at Earth, and blackmailed Maria Esquival into helping him. When he sabotaged a Pelican dropship in an attempt to kill her son, Raphael, she decided to free the newborns. Isto found her and brought her, along with the newborns, to his corvette. When the Shadow of Intent arrived at Earth, and a group of UNSC and Covenant soldiers alike attacked his ship, He nearly killed Rafael, when Rtas 'Vadum found them. Isto was killed by the Fleet Master during a heavy fight in the Control Centre.