Fifth Layer of the Prison of the Multiverse (one of the Planes of Conflict)

Each orb is ~5,000,000mi from its neighbors.

Home to many nameless aquatic lifeforms, the Titan Oceanus in his underwater Temple, a mysterious uncrewed ferry that wanders inexplicably between the orbs, and the souls of self-absorbed humanoids who refused to aid others when they could have trivially done so in their previous lives.

These orbs are covered in shallow oceans of burning acid, with the only dry land found on shifting sandbars, which are desperately contested by this layer's inhabitants. Black clouds float unpredictably between the orbs, heralding caustic snowstorms and making travel between orbs far more dangerous than it is in other layers.

Travel to the previous layer above requires use of portals found atop some orbs' temporarily-highest sandbars. The single portal to the final layer below can be found in the deepest trench of all orbs' seas.

(Based on D&D 2E's Planescape material)