The Darkness

The Darkness or also known as The Black Shadow, is a malevolent force residing inside Yuya Sakaki. This force was presumedly born from Yuto's deep seethed hatred for Fusion Dimension and Duel Academy (ARC-V)

Effect
The Darkness first awakened from inside Yuya when Yuto took over the former's body and initiated the Awakening, 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 behaviour, 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 Awakening 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.

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