The following section contains information from StarCraft: Insurrection.
He was one of Executor Andraxxus' most trusted subordinates, making him an enemy of Syndrea. He did his best to guide the political ascent of his daughter, Edullon, trying to keep her close to him.
Demioch was a member of Tassadar's expedition to the Koprulu Sector, and a member of7th Fleet under the command of Executor Andraxxus. He fought in the early stages of the Great War, where he became close companions with the high templar Xerxes. Known for being a great warrior, conflict undid the body of Demioch, and he was sealed within the shell of a dragoon.
Demioch was part of 7th Fleet's expedition to the terran world of Brontes IV, which was reeling from an invasion by the zerg. With the planet's space platforms having been secured, Judicator Syndrea wished to immediately purify the planet from orbit. However, Andraxxus and Demioch were opposed to this action. Before Andraxxus left orbit of Brontes IV, he told Syndrea to protect the terrans if possible.
Demioch was ordered to the surface along with his daughter Edullon and the archon Aedus/Xerxes, formed from the union between his old friend his old love Aedus. Edullon believed that Demioch's misison, to assault the central hive at Carvalho, was suicidal, and that Syndrea sent him there because, as Andraxxus' friend, he had much influence over the executor. However, Demioch insisted on carrying out the mission.
Demioch did not enjoy the same success as his fellow commanders, and was captured by the rebel group known as the Fist of Redemption, under the leadership of the ghost Atticus Carpenter. He was rescued by Edullon with the assistance of Charlie Vane. Edullon had made an alliance with a rebel terran group named the New Dresdin Outlaws, and Demioch joined with them against Syndrea with the intention of informing Andraxxus of her treachery. Edullon contacted Syndrea and criticized her orders to have Aedus/Xerxes' attack the Incubus Brood's mineral reserves, causing them to frenzy against the protoss and undefended terran colonies. Syndrea, seeing her outspokenness as mutiny, declared her a heretic and ordered Aedus/Xerxes to kill her and Demioch. However, Aedus/Xerxes informed them of the revival of Carpenter as a cerebrate, but stated that when next they met he would have to kill them. Demioch, along with Edullon and the New Dresdin Outlaws, proceeded to attack Carpenter's forces, who were now in control of the Incubus Brood.
Syndrea ordered an attack on Edullon's protoss. Demioch sensed the presence of Aedus/Xerxes, and expressed concern he could not slay his old friend himself in honorable combat. Edullon volunteered to challenge the archon to single combat, and defeat the ancient warrior with honor. The protoss/terran alliance successfully repelled Syndrea's forces. During the battle, Edullon slew Aedus/Xerxes in a duel.
The allied protoss and terran forces had to deal with zerg reinforcements. Formally making an alliance, the protoss and terrans sought aid from Andraxxus. However, the zerg descended upon them en masse when they arrived. All those present were killed, Demioch among them.