Can I permanently banish a devil from one layer of the Hells to another using the banishment spell?
Let's say my party is on Avernus. They are attacked by a devil who's native to Dis (or some other layer of the Nine Hells), and one of the characters casts banishment on it. Are Dis and Avernus considered two different planes, or would the devil be banished to a harmless demiplane for 1 minute?
Best Answer
The devil would be banished to a harmless demiplane for 1 minute
The Nine Hells is indeed considered to be one plane. It is even possible to travel between the different layers of the Hells without any planar travel/magic (see this Q&A); that said, it is also possible to travel to the Abyss (via the River Styx) from the Nine Hells without any planar travel/magic, and the Abyss is definitely a different plane to the Nine Hells, so by itself that point isn't necessarily proof that the nine layers are all part of one plane.
In the DMG, Chapter 2: Creating a Multiverse, sub heading "Outer Planes", pp. 64-66 describes the Nine Hells. The description heavily implies that it is one plane it given how it is described:
However, if we take a look at the Optional Rule: Pervasive Evil (p. 64), it says:
The Nine Hells is referred to a plane, singular, not as a collection of planes. That last part is particularly explicit: "a plane other than the Nine Hells." From this, I think we can safely conclude that the Nine Hells is one plane with nine layers, not nine planes or linked demiplanes or anything like that.
For completeness, I will also include further evidence presented by illustro in this answer, which explicitly refers to planes with layers still being considered to be one plane (from DMG, p. 58):
So yes, casting banishment on a devil whilst in the Nine Hells would banish the devil to a harmless demiplane, not to somewhere else on the same plane (i.e. a different layer of the Nine Hells), since banishment says (PHB, p. 217):