Concept art for Avengers: Infinity War has hinted at just how the Soul Stone may have affected the Red Skull. The unexpected return of the Red Skull was one of the highlights of Infinity War, delighting fans and critics alike. It seemed that he had been stranded on the planet Vormir by the Space Stone, and for decades had acted as the Stonekeeper of the Soul Stone.
But the Red Skull had clearly changed over the course of the decades. He was introduced as a strange, ethereal being, swooping down from the skies in order to meet Thanos and Gamora. What's more, the Red Skull had learned a lot about the Infinity Stones; crucially, he knew how Thanos could acquire the Soul Stone, by sacrificing "a soul for a soul." Avengers: Infinity War never explained how he learned this, although it's possible that knowledge was contained in the Book of Yggdrasil.
Related: Why Marvel Recast The Red Skull For Avengers: Infinity War
The Art of Avengers: Infinity War offers a tantalizing clue, though. It includes a wealth of stunning concept art related to the Red Skull, and concept artist Rodney Fuentebella explained at length how he designed the Red Skull's appearance, originally giving him more of a ghostlike quality.
"I was so excited to work on this character that has been pivotal in the MCU. I was playing with how early the audience should know that this character - previously called the Stonekeeper in the script - is the Red Skull. I designed versions hiding his face and versions showing how much life or soul the stone took from him. I wanted to play with the idea of what the Soul Stone had done to him and whether he is a ghostlike being now or something soulless."
https://imgur.com/a/12adARG
In the comics, the Soul Stone has an insatiable appetite to consume souls. The Red Skull has been in close proximity to it for 70 years, and it seems Fuentebella imagined a scenario in which the Soul Stone has fed upon him for all that time. That explains how the Red Skull has survived on a barren world where there's no sign of any other living thing; there's a sense in which he is no longer truly alive. It also explains how the Red Skull is able to drift down from the heavens; his body no longer has any substance to it. He's literally a shadow of his old self. This may also explain just how the Red Skull knows so much about the Soul Stone's nature; he's bonded with it, and can only survive by serving its purposes.
The Avengers: Infinity War Prelude comics described the Soul Stone as the Avengers' biggest threat, a comment that's yet to be explained. Fuentebella's concept art may well be a tantalizing hint that just being near to the Soul Stone is to risk being consumed by it. It's even possible that Vormir, a planet that has water but apparently no life, was once inhabited. The Soul Stone is believed to have been on that world for millennia, so could potentially have devoured Vormir's life over the ages. If all this is the case, then it has to be asked how wearing the Soul Stone on his Gauntlet will affect Thanos.
Meanwhile, it's important to note that there's a strong sense of irony to the final design for the Red Skull. When the former super-villain descends from the sky in order to meet Thanos, he looks like an avatar of death itself. The Thanos of the comics, of course, has a completely different motive to the MCU version; he's driven by an obsessive love for the (female) personification of Death. The Red Skull's presence serves as a brilliant visual nod to that.
