What is the nature of Tiamat's imprisonment?

Question

Background: In HotDQ, one of my PC's is a Tiefling who grew up orphaned and wandering the Nine Hells. (I find it a little farfetched, but I'm trying not to step on people's creativity unnecessarily.) As a detail of his Hermit background, he gets the "Discovery" feature. He's proposed that somewhere in there, he might have run across Tiamat's prison.

This seems pretty cool to me as something that would tie his character into the story without necessarily giving him an undue advantage... but if we go with it, what are the details? Is Tiamat literally imprisoned, with giant golden chains? Is she metaphorically imprisoned in that she's somehow lost the ability to traverse planes that gods presumably have, but she's free to roam about otherwise? (Or did she never have the ability to travel freely to the Material Plane?) Essentially, where is Tiamat now, and what is she doing while she waits for her followers to summon her?

Answer 1

The information below is taken from Rise of Tiamat, specifically "Hosts of the Nine Hells" (p.10), "Tiamat's Temple" (pp.85-86), and "Victory or Defeat" (p.88).

Tiamat lives in

Avernus, one of the Nine Hells. Specifically, she lives in her temple there, at liberty to move about Avernus. She is not, as you wondered, literally chained to a spot in the Nine Hells.

However, events of the recent past (read: 4e -> 5e, so we've got to undo Sundering and Upheaval!) mean that

"without powerful magic and mortal aid, Tiamat cannot travel from her home in Avernus." (p.10, emphasis mine)

In addition, even if brought to Toril, with the right sequence of events

"the shrieking Dragon Queen disintegrates and is dragged back to Avernus." (p.88, emphasis mine.)

Thus is Tiamat "imprisoned" in Avernus: she needs mortal involvement to travel to Toril, as opposed to the recent past when she could go on walkabout at her leisure. Despite her godhood,

she is bound to Avernus until others actions free her, and can be sent back there.

---

As for what she's up to these days... the one bit we get in RoT is that

Severin impressed her, and that "she allowed Severin a scrap of knowledge regarding the existence of the five dragon masks." (p.8)

So there's obviously some awareness and communication, but no mention of anything else.

Answer 2

The 5e DMG, on the section about the Planes of the Multiverse, specifically on the paragraphs about Avernus (p. 64), states that:

Tiamat, the Queen of Evil Dragons, is a prisoner on this layer, ruling her own domain but confined to the Nine Hells by Asmodeus in accordance with some ancient contract (the terms of which are known only to Tiamat and the Lords of the Nine).

I've never seen any other mention about this contract (neither on previous editions nor in other books of the 5th). So it turns out that the sourcebooks kind of leave the matter open to DM's imagination.

As others have said already: Tiamat is clearly imprisoned (not in actual chains and stuff, but more like enslaved/serving) and can't get out of Baator without aid.

Baator is her home Plane now and she resides in Avernus serving as an advisor to Zariel (the first Lord who lost her position to Bel; then at some point Tiamat was made Lord of Avernus, but she really sucked at it so Bel had the position back; and for some unknown reason Bel was demoted and Zariel was made Lord of the First once again... uff, it's complicated, but it's something like that). Tiamat is also the guardian of the gates to the second layer of hell, Dis.

She's agreed to serve Asmodeus and not to leave the Nine Hells, so the contract must grant her some benefits (maybe she can draw some of Baator's magical power the same way Devils do). Or perhaps she needed some aid/protection ages ago and Asmodeus agreed to "rescue" her if she signed the contract.

The thing is that the terms (and the history) of this contract are not really set, so a DM can pretty much create anything to suit his campaign! :D