The Soulworld was a pocket dimension that existed inside the Soul Stone where souls resided in.
Reunion with Gamora
Following her sacrifice on Vormir, Gamora's soul was transported into the Soul Stone, where she reverted to a child form similar to when she'd first met Thanos. After he successfully wiped out half of the universe's population by snapping his fingers with the Infinity Gauntlet during the Battle of Wakanda, Thanos' consciousness briefly traveled into the Soulworld, where he faced his adoptive daughter after walking over a watery floor.
Gamora, turning to meet her adoptive father, asked if he completed his mission, to which he agreed. When she asked what it had cost him, the Mad Titan solemnly remarked that it cost everything he had. Accepting defeat, the little Gamora smiled at her adoptive father as to comfort him before Thanos' consciousness returned to his body on Wakanda.
Following the Infinity War, Thanos immediately teleported to Titan II, where he retired and intended to spend peacefully the rest of his days. Seeing no further use in the Infinity Stones, Thanos used the Infinity Gauntlet to destroy all of them, thus destroying the Soulworld within the Soul Stone in the process, likely obliterating Gamora's soul too, as well.
- In the comics, the Soulworld, or Soul World, is a dimension covered in a dusty plain which exists inside the Soul Gem and housed the souls of Goddess and Magus.
Behind the Scenes
- The Soulworld was set to appear in a deleted scene in Avengers: Endgame in which Tony Stark meets an older version of his daugher Morgan (portrayed by Katherine Langford rather than Lexi Rabe). The scene was shown in a test screening of the movie and both confused the audience and did not emotionally resonate with them and it was cut for these reasons.