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?