I was hoping someone could help illuminate some D&D lore for me (and I suppose anyone else who decides to ask questions here too).
My question is this; in Baator (= the Nine Hells?) does time flow at the same rate as that of the Prime Material Plane (or indeed, any Plane at all?) or do they follow their own independent time streams.
The official answer to this is "unclear". It can flow at the same rate, or at a different rate. The official books don't take position on this, to avoid hindering your creativity when you plan your own D&D game.
Also; where exactly does the Prime Material Plane 'finish', it is my understanding that it extends far beyond lets say the 'solar system' of Faerun (if that is the planet's name).
Toril is the name of the planet. Faerun is a continent on it.
As for where does the Prime Material Plane "finish", the answer to that has varied a lot as D&D evolved over the years. The current version (5E) has yet to take position on this. However, recent books and even the BG3 itself has been referencing a lot of lore from the AD&D 2nd edition campaign setting Spelljammer (basically D&D in space). Lae'zel straight up references elements from that old lore when she tells you about the time she and her kin successfully raided a "neogi spelljammer" (spelljammer being a type of D&D spaceship and neogi being a evil space-faring race). Crèche K'liir is also a location from old Spelljammer lore. So we know that 5E Material Plane has stars and planets and that at least some Spelljammer lore still applies. Beyond that, we can only speculate.
In Spelljammer lore, Toril's solar system (called Realmspace) encompasses several habitable planets that all orbit Toril's sun and an expanse of unexplored void called Wildspace. All of that is contained inside an astronomically large structure called a Crystal Sphere. Think dyson sphere of immense proportions. If you somehow manage to get outside of a Crystal Sphere (requiring artifacts or very high-level magic), you wind up in a silvery void filled with an iridescent substance called the Phlogiston. Within the Phlogiston are many different Crystal Spheres, each containing its own D&D setting with its own laws of magic and physics. So for example one Crystal Sphere might contain a solar system with planets orbiting a single star, another might have a central world with the sun and moon orbiting around it, while another still might contain a giant turtle carrying three elephants who hold a disk-shaped world on their backs.
The Phlogiston is infinite in size, which makes the Material Plane infinite as well. To reach other planes of existence from here, you need a spell like Plane Shift or a planar portal.
Again, this is old 2nd edition lore, and we don't know how much of it is still valid. But BG3 has referenced several elements from it, so that's something.
I have a bit of a headcanon for a warlock character in that he is his own patron, not that he knows this. The idea being that, born as a tiefling and have grown up being subjected to the prejudices against Tieflings all his life, he begins to believe that he is fated for Hell regardless, so he decides to embrace it. In a self-fulfilling prophecy, he comes in contact with a Devil (those are the lawful evil ones yes?) who makes him promises, and he willfully submits. When he becomes convinced that he would become a Lemure, he embraces his nature even more fully to try and 'work off' this fate and to be promoted, upon his death, to a higher level devil and skip the basic level.
That's a great character concept, and there's actually precedent for this. In the recent 5E adventure Descent into Avernus, player character might end up striking various bargains with the rulers of the Nine Hells. One such bargain has the player character's agreeing to have their soul, should they die after achieving 17th level or higher, sucked into the 9th level of Baator and turned directly into a Pit Fiend (the highest level of devil) to serve as a general in Asmodeus's army. So definitely possible.
