The Vacu were a group of human priests who, according to legend, served the higher demon lords of Gielinor during the God Wars. They are frequently referred to during the Legends' Quest, during which the Karamjan shaman Ungadulu sheds some light on their history and nature.
According to Ungadulu, the Vacu served demon-kind at some point in the past. Given that the demon Nezikchened refers to the player as Vacu, and is believed to have been imprisoned in the Kharazi Jungle during the God Wars, this was likely their time of activity. Little is known about the Vacu themselves. Many demons left the Infernal Dimensions for Gielinor to serve Zaros and, later Zamorak; given that there were many demon lords active at the time, it is highly likely that the Vacu's role in Gielinor were fairly large.