A big concern I have about running the final battle is how a large army is supposed to arrive near the Well of Dragons without being detected?
I plan to have various teleportation circles opened on the plains near Waterdeep that allow the assembled armies to march through and be instantaneously transported to locations near the Well but it doesn't seem likely that a large force can assemble undetected? And thus goes the element of surprise I would imagine. So I feel like the plan has to be made in Waterdeep and then the teleportation is step one of the days campaign (i.e. there's no staging, just attack, attack, attack).
I guess I could have one small teleportation circle on the rim of the caldera which has been prepared for the PC team (+ a few helpers). They will teleport there and wait for the main assault to begin in order to gain some cover from the chaos.
So once the final push begins the PCs will make their way across the caldera to the Temple (I'm not going with the underground caverns - that seems far less epic than this grand finale deserves). I want them to witness the clash of dragons in the air above, the cultists panicking and trying to defend the temple from the onslaught. Just general mayhem and the PCs cutting their way through.
Mounted on the temple will be the Draakhorn making its baleful moan which becomes more strident as the ritual proceeds. (assuming they leave it intact).
Beneath the caldera is where the prisoners are kept - they're in a single file being marched up to a platform in front (but to one side) of the temple and being dispatched one by one and their lifeless bodies being tossed into a fiery lava pit.
The doorway leading into the temple stands open below the Draakhorn platform and arcane light can be seen flickering within. The sky above is thick with swirling storm clouds of course - giving the whole seen an eerie half-light.