Realms of Fate

The Realms of Fate are a sort of 'portal world,' which connect various locations in Sanctuary and beyond.

Lore
The Realms of Fate are the unintended consequence of the battle that took place between the Nephalem and Diablo in the High Heavens. Their existence was discovered by Itherael, as he observed that tomes within the Library of Fate were changing Heaven itself, transforming into small pockets reflecting the past, present, and potential futures. The pockets appeared within the skies of Heaven. Though none could make sense of why this had occurred, this phenomenon still posed a threat to Sanctuary. Tyrael invited the Nephalem to investigate.

In-game
The Realms of Fate were added to the Adventure Mode of Diablo III in patch 2.6.0.

They act as an artificial mesh-up of zones. For instance, a player may be in a location that mirrors Westmarch, and find themselves walking into a mirror of part of the Pandemonium Fortress.

Each of these zones serves as a separate location in. These can only be accessed in Adventure Mode through Waypoints, which first place the Nephalem into a small section of Library of Fate, and then shift into other zones seamlessly. Each zone contains monsters typical for this layout, and has several Unique Monsters that can only spawn in these reality warped sections. A bounty in a Realms of Fate zone is typically to complete a randomized event (such as The Black King's Legacy), cleanse a Cursed Chest, or slay a Unique monster.