The Darkness

The Darkness, also known as The Black Shadow, is a malevolent force residing inside Yuya Sakaki.

Effect
The Darkness first awakened from inside Yuya when Yuto took over the former's body and initiated Berserk Mode, resulting the Darkness to spill into Yuya as well. The Darkness influenced Yuya and Yuto's Dueling style to become battle hardened and violent. Not only that, it also influenced Yuya's behavior, making him malicious with only desire to defeat his opponents and takes pleasure in causing and seeing harm to them, particularly Duelists from Duel Academy due to its birth from Yuto's anger and hatred for them. The Darkness also strengthened Yuya's Berserk state more than usual that signed by a thick black aura surrounding Yuya's body. Consequently, while under its influence, neither Yuya nor Yuto are aware of their surrounding and if stopped by someone while not Dueling, Yuya became more unstable to the point of needing to be restrained until he finally reverted back to his normal self. When in a synchronized berserk state with all of his dimensional counterparts, they also gain dark auras around themselves as well until the synchronization ends.

Arc League Championship
The Darkness first appeared during Yuya's duel with Iggy Arlo, but was only recognized as a separate entity when Aura used her crystal ball on the out of control berserk Yuya while he was restrained by Gong, Reed, and Trout. Through her crystal ball, it was shown that the Darkness slowly tried to overwhelm both Yuya and Yuto's souls while they were still in their berserk state, though Yuya fell unconscious from his berserk state before it could do so. Once Yuya regained his consciousness and realised its existence as well as Yuto's soul inside him, Yuya came to the realization that the Darkness was the one responsible for his violent Dueling against the Obelisk Force. This made Yuya scared for a moment, but he quickly dismissed it by remembering his father's encouraging words.

Friendship Cup Finals
The Darkness returned during Yuya's synchronized Berserk Mode with Yuto, Yugo, and Yuri after Roger activated the chip in Yuya's helmet. It gave Yugo and Yuri dark auras around them when Yuya Summoned "Odd-Eyes Rebellion Dragon". The Darkness disappeared again after Yuya and his counterparts' synchronized berserk state when they defeated Crow Hogan while the chip self destructed in the process.

The Darkness made its return again when the Four Dimension Dragons sent Yuya and his counterparts into a synchronized Berserk Mode after all dragons appeared on the field. However, the Darkness was suppressed once again after Zuzu arrived and her bracelet teleported Yugo and Yuri away while sending Yuya flying into the ceiling.

Heartland City
Yuya and Yuto acknowledged the strong control the Darkness had over both of them each time they were in Berserk Mode. Yuto recalled the Four Dimension Dragons' desire to become one and suspected that they have a strong connection to the Darkness. He also theorized that his current residence in Yuya's body may have had to do with "Dark Rebellion". During Yuya and Shay's Duel against Gloria and Grace Tyler, Yuya was worried about Yuto's anger controlling him because the Darkness might return to possess them again.