Avicus is a vampire character, one of the oldest in The Vampire Chronicles.
Avicus is far older than Marius, but he does not know his own strength or how to use his inherent gifts, but it is implied in Blood and Gold that his powers are second to Marius. Avicus is from Egypt and was made a vampire by a priest from Akasha's cult. He was also allowed to drink from Akasha herself and her powerful blood coupled with his age gives him his great strength.
When the druid priest Mael needed a new God of the Grove due to Marius's escape and the destruction of the previous God of the Grove it was decided that the position would be fulfilled by Mael. They learned about Avicus, who was said to be found in England, and traveled there; though it is indicated that the forests to the north were most likely in Scotland, which at the time when the events take place had a large druid presence and was completely covered in the Caledonian forest.
Mael and Avicus makes an arrangement: Avicus will give Mael the Dark Gift if he is given a victim and freedom. Avicus wants to escape after learning from using the Mind Gift on Mael that Marius successfully escaped. Ultimately, Avicus and Mael escape together and become companions. They would later find Marius and live together for a time.
In Constantinople, Marius, Avicus and Mael discover Zenobia, a vampire fledgling of the ancient vampire Eudoxia. Mael, Avicus, and Zenobia are left behind by Marius when he takes Those Who Must Be Kept out of Constantinople after having Eudoxia killed by Akasha.
Over 1,000 years later, Marius learns from Mael that Avicus was in love with Zenobia and he persuaded her to leave with him, abandoning Mael. Mael blames Marius for this, as he blamed Marius for all his misfortunes in their past.