That's consistent with the lore about the Nine Hells in general. No devil can leave the Hells on their own. They all must be summoned and so forth. I doubt even Asmodeus can leave of his own free will.
That's more in line with demons, which need to be summoned. Devils have been presented as being able to leave more freely in order to make deals.
Even then, it'd only really apply to devils. Other outsiders, like night hags and rakshasas, don't seem to be as limited. So why would any of the many gods who have dwelled in the Nine Hells be imprisoned there? (The gods of the goblins, orcs, sahuagin, and kobolds have all lived in the Nine Hells at one point.)
And it certainly doesn't mesh with the idea of the Blood Wars, unless the demons and devils were summoning each other...
To say nothing of the fact the 4e Realms campaign setting has Tiamat move into Banehold with Bane.
The "devils need to be summoned" thing is reasonable headcanon, and a decent justification. But it still doesn't mesh with the "imprisoned" line unless anything that goes into the Nine Hells can't leave.
My problem is the book never bother to explain or even attempt to justify it, when all it took was a single sentence. "Tiamat angered Bane after a failed assassination of Bahaumat and was imprisoned in the Nine Hells." That's
literally all it would have taken. But they couldn't even bother doing that. Or just not refer to her as being a prisoner and instead they're just trying to summon her into the mortal world.
But zero effort was made. They just did the story and decided "it makes more sense if Tiamat is trapped. Guess she's been trapped. And has been for a "long time"."
Which is why I'm hesitant about Acererak suddenly becoming a lich. And not lich a lich, but supposedly the most powerful lich. (Ummm... Vecna?) But Acererak
isn't a lich. He's the iconic demilich. So I'm understandably worried he might have suddenly changed into a lich for no reason and that the adventure won't even try to explain it.
Which bugs the crap out of me. Changing canon for no good reason without even a token handwavy excuse is extremely frustrating to me.
I can't wait for the next adventure when Elminster is a 14-year-old girl. The gith/ Mind Flayer war adventure, where the gith are ruled by a giant gith golem. And the Modron March adventure where Primus looks like a giant robot planet.
Anyhoo... this threads about
Tomb of Annihilation and I should probably stop filling it with my Tiamat rants...