The party first encountered Altair while fighting in A Grand Arena Tournament to earn their freedom. After they defeated him and earned their freedom, he recruited them to help Orion investigate a demonic cult.
He was also a part of Aiden’s plan to kill Morpheus. His part being that he was to kill Aiden while Morpheus possessed his body, and to negate Morpheus’s ability to leave the body. Which would result in Morpheus’s death. Unfortunately for Aiden, this plan failed when Morpheus became a Vestige instead of dying.