The writers of Avengers: Endgame have divulged some insight into how Red Skull wound up appearing during Avengers: Infinity War.
During a panel at Comic-Con International in San Diego, Stephen McFeely and Christopher Markus recalled the moment they realized that Red Skull was not only still alive, but a character they could utilize for Infinity War.
"There's a fascinating one-act play to be written of Captain America going to Red Skull," Markus joked. "Never wrote it. In the manifesto document, there was one theory that Cap who goes to space... and he has to collaborate with Red Skull to get the Stone."
On the topic of Red Skull, Markus revealed Marvel Studios composed a "manifesto" to explore options for Endgame from the earlier films. Red Skull was one of them.
"We wrote a big document that was everything you could possibly do in these movies," he revealed. "In there, the Red Skull is clearly alive... He's awesome! Let's find a way to use him. Let's tie this whole thing together... We did send him to space and send him away very much on purpose..."
He noted that they didn't know where he was going when Red Skull disappeared in Captain America: The First Avenger.
During Infinity War, Thanos traveled to Vormir with his adopted daughter Gamora in his attempt to attain the Soul Stone. It was in this scene that fans were reintroduced to Red Skull.
Red Skull's arc in The First Avenger culminated in his desperate use of the Tesseract during the blockbuster's climactic scenes. A wormhole tore into their reality, sucking him through, leading to his disappearance from the MCU until 2018's Infinity War.