Kaleidoscope and Mystery Dungeons

A continent-spanning dungeon beneath the Sword Coast. Has an entrance in the crypts beneath the Obaskyr castle in Arabel.
or so you might think. What's underneath the Obraskr Castle is something called a Mystery Dungeon.
To understand this, we must start from the top.


The Kaleidoscope is just another name for The Plane of Memories, and within it is also The Demiplane of Time which houses the history of the Forgotten Realms within a Grant Tower called The Temporal Tower, or more well known amongst arcana fanatics (Netherse Empire) as The Akashic Records.
Consider the Plane Wheel Cosmology. For simplicity's sake let's assume the Inner Planes like the Material Plane & The Elemental Planes are on the x-axis, and the Outer Planes like the Nine Hells & Mount Celestia lie on the y-axis. Each of the Planes are connected but seperate with a creature being able to travel between them through the "gaps" (The Ethereal Plane, The Astral Sea, or the Elemental Chaos) between them.
The Kaleidoscope on the other hand is a Plane of Existence which lies on the z-axis, as it is both part of each of the Inner and Outer planes of existence, but also it's own seperate dimension with much of it being completely theoretical. As in, It is possible to extract information and confirm it's existence, but it is impossible to traverse it in a very literal sense. 
When the Kaleidoscope interacts with certain aspects of reality, it mixes and blends with it which is the only way for a being to physically see this place. Wherever the Kaleidoscope meshes in with the concept of Time, that area is then considered part of the the Demiplane of Time. Similarly, whenever The Kaleidoscope interacts with magical hotspots within the Inner or Outer Planes (hotspots such as Leylines), it creates a pocket demiplane where memory of the world and reality blend in together. This blend is called a Mystery Dungeon.
Simply put, a Mystery Dungeon is a magical hotspot that causes tears between that Plane of Existence and The Kaleidoscope. Whenever this occurs, a link is formed between the two planes, allowing for a creature traversing through that area to become part of that memory, almost as if they are travelling through time. However, as what is being shown/happening is just a memory, it is outside the realm of causality since it's not literal Time Travel.
The Two known Mystery Dungeons (to the players) are underneath Arabel (seen: The Starsent) and underneath The Peak of Flames (seen: Repel Tartarus)

Notes from a researcher by the name of Dr. Barry B. Boltorbson, expanded upon by three other co-researchers titled Vodka Martini, Ginnungagap, and C.C.