Acastus was a Tok'ra operative.
He worked for Queen Helen's household on the planet Aegis. A Half-century ago, Helen became aware of a Helots named Acastus who worked in her household. Helen then discovers his true identity to Acastus is actually a Tok'ra operative. After a long night of discussions, he could convince Helen to just the Tok'ra organzation with Helen agreeing to keep their secret.
As time passed, their relationship deepened and they became lovers. They did their best to keep it a secret, but Alexander discovered their relationship and tried to kill Acastus. The symbiote Acastus survived but his host body was too badly injured to live.
He asked Helen allowed to mix with her, and she went. Unfortunately trauma was too great. Acastus survived a day after the mixture. His death was blow her but she was comforted by the remaining memories of his life. He never got to meet his son who Helen was with him. (RPG: "Stargate SG-1: Living Gods: Stargate System Lords")