Devack, known as Demak in the Japanese version, was one of the Dark Signers. He freed Greiger from his prison transport, and offered him the chance to fulfill his desire for vengeance. He was also responsible for the Minus Curse that infected the Spirit World with the help of his subordinate, Zeman The Ape King.
Devack wears the black robes of the Dark Signers that were accented with yellow stripes. His dark mark is the monkey.
Devack had great loyalty to the Dark Signers and to Roman in particular. His final words were an apology to Roman for losing. He also looked down on those who were not Signers, and enjoyed the pain of others, as seen when he tried to show Luna more sadness by using "Ancient Fairy Dragon" against her.
It is unknown how Devack died and how he became a Dark Signer, or what grudge he had that allowed him to be revived in the first place. At some point upon his rebirth, he met Roman and the other Dark Signers, and pledged his loyalty to Roman. At some point, he was given Ancient Fairy Dragon by Roman to set up his eventual confrontation with Luna. He appeared out of the original Ener-D Reactor by arriving by helicopter in order to free Greiger from custody with an "Axe of Despair" card, as his transport guard's souls had been absorbed by the "Earthbound Immortal Ccaryahua". Devack then invited him to join them and get revenge on Rex Goodwin for the destruction of his hometown. They later picked up Misty Tredwell and Carly Carmine from the destroyed Arcadia Movement building and returned to the Ener-D reactor.
He finally showed himself to his rivals, the Signers, after Roman's Shadow Duel against Yusei concluded. Before their departure, he showed Luna that he possessed the "Ancient Fairy Dragon" card, and so in order to get it back, she must fight him. Later on, as each one of the Signers was set on defeating their respective opponent, they split up to each of the four control units of the Original Ener-D reactor. As Luna and Leo were taken by Trudge to where Devack was, she instead went into the Duel Monsters Spirit World in order to free Ancient Fairy Dragon from her imprisonment with the the help of her servant, Regulus. However, when Leo found the control unit, Devack approached him while the Monkey geoglyph materialized. Leo then bravely challenged him, and stating that if he defeated the Dark Signer then Luna wouldn't have to Duel him.
Despite Devack's warning that Leo wouldn't be able to save himself and his sister, as he was not a Signer, the Duel was still on. Devack was quickly able to inflict damage to his opponent at the start, but Leo counterattacked with "Power Tool Dragon", his ace card. Devack then Summoned the Dark Synchro Monster, "Zeman the Ape King". Eventually, Leo was able to destroy his Dark Synchro Monster, at the exact same time that Regulus destroyed Zeman in the Spirit World, by utilizing his "Power Tool Dragon". However Devack Summoned his true ace card, "Earthbound Immortal Cusillu". He attacked directly, but Leo prevented himself from being defeated. Then, Luna arrived, and took Leo's Duel Disk and the twins Dueled together.
However, Devack Summoned "Ancient Fairy Dragon", and later sacrificed her to prevent the destruction of "Cusillu". This ultimately backfired for him, as Luna used Leo's "Respect Synchron" to Summon "Ancient Fairy" to her side and used its effect to destroy "Closed Forest" thus negating all the effects of "Cusillu" and rendering it useless. Then Luna assaulted it with "Power Tool Dragon", weakening it and then destroyed it with "Ancient Fairy Dragon". The Duel ended with Devack's defeat.
Regretting his defeat, Devack turned into dust while asking Roman for forgiveness for failing the Dark Signers. (In the dub, he tried to drag Leo and Luna with him to the Netherworld, but failed when Yusei came in and saved them).
When Rex was defeated in the final battle and the King of the Netherworld was destroyed, Devack was restored to life, along with the other Dark Signers except for Roman and Rex himself. It is unknown if he kept the memories of his time as a Dark Signer.
Devack was the only Dark Signer whose death and transformation into a Dark Signer was never explored.