Warning: SPOILERS for Infinity Wars: Sleepwalker
Marvel's Infinity Wars comic event has permanently changed the cosmic landscape, revealing that every single one of the Infinity Stones, not just the Soul stone, contains its own world. It's the greatest secret of the Infinity Stones, rewriting Marvel Comics lore in fascinating new ways.
It's long been known that the Soul Stone contains a mysterious realm known as the Soul World, recently seen in Infinity War. In the Infinity Wars comic, for those who haven't been following issue by issue, Gamora has used amplified power of the six Infinity Stones to plunge all of reality inside the Soul Stone. And while Gamora doesn't know it, her actions has tipped the nature of the universe out of balance... and someone has noticed.
Infinity Wars has already shown that the Infinity Stones are like a circuit, with power flowing from one Stone to the next. The Infinity Wars: Sleepwalker has added another detail to that concept, revealing that each Infinity Stone contains its own Stone World. It's actually possible to move from one Stone World to the next, but only by mastering the Stone World you are currently in. Some of these Stone Worlds are higher planes of existence already introduced to Marvel comics, like the Mindscape. Thus Sleepwalker, a creature of the Mindscape, has begun a quest to get into Soul World.
In order to do so, he must traverse the full circuit, and move from one Infinity Stone's World to the next. Only then that he can make his way into the Soul World and attempt to right the cosmic balance. It's a fascinating story, and it also has the added value of introducing readers at last to every single one of the Stone Worlds. Allow us to provide the tour...
The Mindscape: The World Inside the Mind Stone
The Mindscape is a psychic realm populated with the thoughts of every living being in the universe. It's populated by all the creatures the mind can ever conceive, with nightmare beings and demons of Despair, Paranoia, and Depression all given manifest form. The Sleepwalkers police the Mindscape, and their job is to defeat and apprehend the creatures that would seek to possess humans. There appears to be some sort of link between the Mindscape and the Fear Dimension inhabited by Nightmare, but the exact nature of that links is as yet unclear. The Mindscape has existed in the Marvel Comics universe since 1991, when the Sleepwalkers were first introduced, and it's fascinating to see Marvel turn it into one of the Stone Worlds.
Sleepwalker deduces that it's possible to travel from one Stone World to another. He realizes that Gamora's actions are her dream - one that is becoming a nightmare, as the nature of reality fractures and begins to break down. That dream, that nightmare, can serve as a doorway to the Sleepwalker by which he travels to the next Stone World - the world inside the Power Stone.
The Arena: The World Inside the Power Stone
The world inside the Power Stone is known as the Arena, a place of unlimited combat where all things vie to be the champion. The greatest being in this World Stone is Dynamus, who calls himself the "living embodiment of the Power Cosmic." It's unclear if he's a permanent inhabitant of the Arena, or one of Gamora's twisted creations; visually he appears to be a combination of the Silver Surfer and Adam Warlock, which suggests that he was made by Gamora to serve as master of the Power Stone. There are only two ways out of the Arena; to be defeated and die, or to triumph and leave. Fortunately, the Sleepwalker remembers that - as mentioned - the Infinity Stones are a circuit. To be master of the Power Stone, you must be a creature of pure thought; and as just such a being, he is able to manipulate the Arena itself around Dynamus, and thus defeat its champion.