Looking for ways to link the Tyranny of Dragons setting into leading the players to enter the Undermountain in an effort to thwart the efforts of the Cult of the Dragons raising Tiamat. 

My thought is that when they engaged Skyreach Castle and deal with Rezmir and find that the chest is empty where the last Dragon Mask has teleported to a different location. That they determine that this mask is somewhere within the bowels of Undermountain where there is now a race to locate this last dragon mask which should motivate the Cult to enter the bowels of Undermountain to locate this artifact....

Suggestions, ideas?

Your idea sounds good.

A idea that just came to me would be if at the end of tyranny the players fail to defeat Tiamat. She is loose and soon will set about destroying the world. The players learn that good old Halister has a magical device that they can go back in time to try again. They would be higher level so maybe they could speed run undermountain, at least tell the lower levels?

sounds very interesting.....doing alot of reading to put it all together. We start this Saturday. Should be fun