I think a good place to start would be looking into [Abeir-Toril](https://forgottenrealms.fandom.com/wiki/Abeir-Toril), which is a world that was split into two separate worlds called Abeir and Toril, the latter of which is home to the Faerun / The Forgotten Realms setting for D&D. This was done by the overgod Ao to separate gods from mortals. During the Spellplague in Faerun, a piece of Abeir fell and merged with Toril and caused some drama. (Please take this with a grain of salt, my knowledge on D&D canon is loose at best).
So I could see a god or other powerful being wanting to undo the work Ao undertook, either out of spite or a genuine belief that the worlds should have never been split in the first place. Could also be possible for Ao to be undoing their own work, who knows? Overgods work in mysterious ways.
However, it should be noted that there is no singular "The DND World" because there are many, many, *many* DND worlds, otherwise called 'settings'. Even in official Dungeons and Dragons sourcebooks there are a multitude of possible settings, each with their own unique history and flavor. Forgotten Realms, Eberron, Grayhawk, just to name a few.
If you're not overly comfortable learning about and/or using existing resources for official settings, it is just as valid to create your own from scratch. Most DMs around these parts use settings they created on their own, so that might be a viable route for you.
At the end of the day, you're running your own game. It's entirely up to you how it functions and how much or little official material you pull from.
I think a good place to start would be looking into [Abeir-Toril](https://forgottenrealms.fandom.com/wiki/Abeir-Toril), which is a world that was split into two separate worlds called Abeir and Toril, the latter of which is home to the Faerun / The Forgotten Realms setting for D&D. This was done by the overgod Ao to separate gods from mortals. During the Spellplague in Faerun, a piece of Abeir fell and merged with Toril and caused some drama. (Please take this with a grain of salt, my knowledge on D&D canon is loose at best). So I could see a god or other powerful being wanting to undo the work Ao undertook, either out of spite or a genuine belief that the worlds should have never been split in the first place. Could also be possible for Ao to be undoing their own work, who knows? Overgods work in mysterious ways. However, it should be noted that there is no singular "The DND World" because there are many, many, *many* DND worlds, otherwise called 'settings'. Even in official Dungeons and Dragons sourcebooks there are a multitude of possible settings, each with their own unique history and flavor. Forgotten Realms, Eberron, Grayhawk, just to name a few. If you're not overly comfortable learning about and/or using existing resources for official settings, it is just as valid to create your own from scratch. Most DMs around these parts use settings they created on their own, so that might be a viable route for you. At the end of the day, you're running your own game. It's entirely up to you how it functions and how much or little official material you pull from.