I've been searching on Google, and the closest thing I've been able to find has been 3.5e homebrew. I want to see if there is anything official for this that I am missing, or something that wouldn't be too difficult to tweak into what I need before I try making something up.
Basically there is a small village, a castle, and a lake that are moved from point A, to point B, and then back.
I think I figured it out. It could be done using a Zaratan and a teleportation spell.
This wiki (Which cites the Arms and Equipment Guide 3rd edition) talks about Zaratans' being capable of supporting small villages.
Older zaratani were comfortable supporting small villages so long as the inhabitants did not disturb their slumber. A few corsairs even used them as hidden bases. The most famous of these was Jayani al-Jasir.
Many inhabitants of zaratan islands were incredibly superstitious. They believed that their island was a deity whose wrath would be terrible if disturbed. Large quantities of fish, and even visitors, were sacrificed to please the zaratan.
The only 5e source I've found doesn't explicitly state that you can live on a Zaratan, but it does support that they are gargantuan and that their shells are capable of supporting a "landscape":
When a zaratan is summoned from the Elemental Plane of Earth, the ground rises up to take the shape of what looks like a hulking, armored reptile, its shell composed of the landscape from which it arose. [...] ZARATAN Gargantuan elemental, neutral
(Page 201, Mordenkainen’s Tome of Foes)
The lake is artificial, is a dip in the zaratan's shell, or is just not part of the teleportation (perhaps there are similar lakes in both locations, perhaps the zaratan lives in one of two large lakes that it is sometimes teleported between)
As long as the Zaratan is alive and willing, the Teleport spell can be used to teleport it, the village and castle (which are part of it's shell), and up to 7 willing residents to the new location.
Teleport
This spell instantly transports you and up to eight willing creatures of your choice that you can see within range, or a single object that you can see within range, to a destination you select. If you target an object, it must be able to fit entirely inside a 10-foot cube, and it can't be held or carried by an unwilling creature. The destination you choose must be known to you, and it must be on the same plane of existence as you.
Notes:
Barovia got stitched into the Demiplane of Dread. Various bits of the feywild and/or prime material got ripped out of their plane(s) and joined to Avernus. Consequently, there is a basis in Lore for locations moving around.
There is no spell nor creature ability yet published that allows territory to move from one place to another. Presumably some must exist, but the DM must create them or find suitable non-first-party material.
I haven't found anything official about relocating such large areas of land for 5e. Your options look to be limited to:
The Wish spell will work as long as it is worded sufficiently and the DM doesn't want to mess with you too much. So a viable option but requiring a high level spell.
A homebrew spell is probably also going to be of a high level as it is a powerful effect you're trying to replicate, even if it is more targeted than the Wish spell.
A divine act is something that could occur at any point of the PC's storyline. They don't need to be able to cast level 9 spells or the like for a God or similarly powerful entity to decide to relocate a chunk of real estate. The why's and wherefores of them doing that I assume you have figured out already.
You can always use Wish, altought it's a high-level spell and risky.
Also, Teleport let's you teleport an object up to 10-foot length, this may include land.
