The entry on devils (p. 68) in the Monster Manual states the following:
"Devils that die in the Nine Hells are destroyed forever – a fate that
even Asmodeus fears."
This makes it clear that not even archdevils or gods are exempt from this rule, and even a god that dies in the Nine Hells has no chance of coming back. My question is, does this apply to Tiamat as well? Though she was not originally a devil, and is technically a dragon god, she once ruled Avernus. This gave her the title of at least Archfiend, and an old article (“Leomund's Tiny Hut: Rearranging and Redefining the Mighty Dragon” by Lenard Lakofka) says that she was an Archdevil.
It's possible that being given this role technically made her a devil. Something similar happened to Zariel, who was previously a solar, but was definitely turned into a devil when she declared her loyalty to Asmodeus and given rule of Avernus. Still, while Zariel's stat blocks describes her as a devil, Tiamat's only describes her as a fiend.
There's also this quote from the newest adventure "Descent Into Avernus" (p. 126) which suggests this permanent death rule may apply to others outside of devils as well.
Mahadi doesn't believe in taking unnecessary risks, particularly in
the Nine Hells where he can be permanently slain.
The mentioned creature is not a devil, but a rakshasa, which makes it curious that he can permanently die here.
I realize that as a DM, I can decide which way to go, but I'd like to know which conclusion should be considered canonical. I don't know if there's anything in Forgotten Realms lore that I've missed that supports one argument more than the other.