Are the Nine Hells/Baator considered a single plane of existence in D&D 5e, or are each of the nine layers considered their own separate planes?
For example, would the spell Scrying, which requires the target to be on the same plane of existence as the caster, function if the caster and target are in separate layers, say one in Avernus (first layer) and the other in Dis (second layer)?
The Dungeon Master's Guide lists the Nine Hells as a single plane in Chapter 2: Creating a Multiverse (page 58). It also often mentions the home plane of devils, without ever specifying a level for any of the devils.
Mordenkainen's Tome of Foes counts the Nine Hells as a single plane. In Chapter 1 The Blood War (page 5):
To the good fortune of the rest of the multiverse, almost all the battles in the Blood War take place in the Abyss and the Nine Hells. Whether by cosmic chance or the design of some unknown power, the dark waters of the Styx provide passage between the two planes, but pathways to other realms are at best fleeting and unreliable.
(emphasis mine).
Descent into Avernus also refers to the Nine Hells as a single plane. In Chapter 3 Pervasive Evil (page 79):
Evil pervades the Nine Hells, and visitors to this plane feel its influence.
...the creature's alignment reverts to normal after one day spent on a plane other than the Nine Hells.
Prior editions of D&D went into far greater detail on the layout of the Outer Planes. Specifically, there each “plane” could have multiple “layers,” each layer separate from the other (requiring plane shift to transit between them unless you used a specific connection).
So the plane Baator had nine layers, hence its other name, the Nine Hells. Mount Celestia had seven layers (so yes, “seventh heaven” is canonically a thing in D&D). The Abyss, notoriously, had infinitely-many layers, each one of them infinite in expanse. Bytopia had two layers that “faced” each other, so looking up from one, you were effectively “looking down” on the surface of the other layer.
