The Truth of Zero Reverse & Dark Signers: Seventeen years prior, Dr. Fudo (Yusei’s father) directed the Ener-D Reactor project. Upon realizing its instability, he tried to halt it, but his assistant Rudger Godwin hijacked the project. Rudger purposefully triggered a catastrophic explosion known as "Zero Reverse," which tore Domino City apart, creating the island of Satellite and killing Yusei’s parents. Rudger and his followers became "Dark Signers"—resurrected corpses possessed by the Earthbound Gods seeking revenge on the Signers.
The True Threat: Iliaster & Z-ONE: In the second half, the mysterious organization "Iliaster" is revealed not to be ancient cultists, but time travelers from a ruined future. In their timeline, humanity’s negative emotions caused Ener-D to overload, triggering a global genocide by autonomous robotic weapons called Meklord Emperors. The ultimate mastermind, Z-ONE, was a scientist in that future who underwent cybernetic surgery to physically transform himself into Yusei Fudo. He hoped that adopting Yusei’s image would inspire humanity to abandon greed and stop Ener-D, but he failed. Left as one of the last living humans, Z-ONE sent his android avatars (Aporia, Antinomy, Paradox) back in time to drop the "Ark Cradle" floating citadel onto Neo Domino City to destroy the original Ener-D reactor and alter history, even if it meant killing millions in the present to save the future.
The Finale: Yusei achieves "Limit Over Accel Synchro" and summons Shooting Quasar Dragon, convincing Z-ONE that humanity can overcome its dark fate. Touched by Yusei’s resolve, Z-ONE sacrifices himself to stabilize the falling Ark Cradle. In the epilogue, the Signer marks fade. Team 5D's disbands as adults: Jack pursues world championship racing, Crow joins the international police, Akiza becomes a doctor, Leo and Luna move abroad, and Yusei stays in Neo Domino City as its lead scientist to ensure Ener-D is safely harnessed for future generations.