Are the Tiefling subraces from Mordenkainen's Tome of Foes specific to a particular campaign setting?
In Mordenkainen's Tome of Foes (MTF) there is a section on Tiefling Subraces (p.21), which elaborates on how the information about a Tiefling's ability score increase and the Infernal Legacy contained in the Player's Handbook relates to Asmodeus. Let's say the "default Tiefling" prior to MTF.
Under Tieflings Subraces there is a variety of Subrace Traits for e.g. Tieflings associated with Baalzebul or Fierna.
It says this is a DM's option to include other Tiefling subraces, but I was wondering: Is this intended specific to a particular Campaign Setting, or it's meant to be a global change, i.e. for all campaign settings?
Best Answer
These variant subraces are not connected to a specific campaign
Mordenkainen's Tome of Foes is not connected to any specific campaign, and the first few chapters are generic "global" changes (emphasis mine to draw attention to the generic language):
That being said, it is ultimately up to the DM whether tiefling of other lineages use the different statistics of these variants or just use the default tiefling in the Player's Handbook (emphasis mine):
On the Cosmology
In D&D lore, Mordenkainen is from Greyhawk:
So far, Greyhawk's only official campaign book in 5e is Ghosts of Saltmarsh which makes no explicit mention of tiefling variants. (Greyhawk was the ‘official’ setting of Advanced D&D)
The Nine Hells (along with other planes discussed in the book, like the Feywild and the Astral Plane), are the same locations in reference to any of the many D&D worlds (or crystal spheres according to Spelljammer). This is only the official cosmology based on past editions and designer comments, though.