Maybe I just need to get and read the tyranny of Dragons books, but according to reviews:
[sblock]it's the Cult of the Dragon trying to help Tiamat win? Thing is, they've never been associated with Tiamat before, they've always been focused on making all dragons Dracoliches, and when they lost Sammaster they even picked up a Dracolich as their leader. Switching allegiances to Tiamat just makes no sense to me; plus, Ed's written about Tiamat before, but she's made almost no impact in FR products before. Reading several reviews, it just feels so shoehorned to me.[/sblock]
The reason I like it so much is because it's brilliant - the reasoning behind it works really well.
[sblock]The Cult of the Dragon was always based on an erroneous reading of a prophecy. The relevant passage is generally translated:
"naught will be left save shuttered thrones with no rulers but the dead. Dragons shall rule the world entire"
except by the cult, which has it as:
"naught will be left save shuttered thrones with no rulers. But the dead dragons shall rule the world entire"
So, Dracoliches. And thus the Cult has continued for the past several hundred years.
Tyranny of Dragons has one of the rising stars of the cult think, "Hang on, this isn't working - we raise these dracoliches, and things still go wrong", so he goes back to the prophecy, and realises that perhaps the cult's founder had it wrong. He's a charismatic chap, and he manages to persuade a bunch of other cultists that perhaps they should be helping living dragons rule and not the dead ones. And Tiamat notices him and starts offering help, in particular, the five masks of the dragon.
And so, with these major artifacts, he gets even more of the cult under his control, and starts planning the dominion of dragons, which will culminate with the release of Tiamat from the Nine Hells. The adventure talks about her being imprisoned there, which actually fits not that badly - as you said, it's not like she's been very active on Faerun - but as I like gods having real trouble coming to the Prime Material Plane, it works fine with me in any case.
What's really nice about the adventures is that although the new cult leader has most of the cult going with him, there are a number of old-timers who think he's nuts and want to go back to the good old days of making dracoliches![/sblock]
Cheers!