This Fallen One is actually the Silver Surfer, the herald of Galactus that Thanos believed he had already killed. It turns out the Surfer survived and became even more powerful than before.

The Fallen One served Thanos until he was killed by two cosmic gods ... That herald is the Silver Surfer. The Silver Surfer is looking decidedly darker than his usual shade of Silver.

Goodbye Thanos, hello Doctor Doom ... when he steals the Power Cosmic from the Silver Surfer and wreaks havoc. This was adapted, poorly, in the 2007 film "Fantastic Four: Rise of the Silver ...

“No one kills Thanos but Thanos ... the Hulk (treated as Thanos’ dog), and the threat of a vengeful Silver Surfer armed with Mjolnir. King Thanos brought his younger self over to help ...

the Silver Surfer wielding Mjolnir. The Cosmic Rider is dead, and so is the Hulk. The Annihilation Wave has been extinguished, and the only matter left is for the Thanos of our time to kill King ...

This is really a swerve because he plans to have them all killed off as a distraction so Silver Surfer can sneak by and steal the Gauntlet off Thanos’ hand. Meanwhile, Thanos’ whims have ...

In the series, Silver Surfer, whose memories of his life as Norrin Radd on the planet Zenn-La were restored by Thanos, protects Earth from Galactus, his former master who seeks to devour the planet.