Things to make the final chapter in a campaign tense

[Cheesy74]

I've got the plot angle of this down: the main villain in my campaign is Father Llymic from the Elder Evils book, and the nations of the world (under my players' direction) are entering a massive war against his brood spawn. My players and a few elite allies from the various nations are heading into his tomb to take the man himself out. The issue I'm having is this:

1. How do I convey Llymic's near-omnipotence within his tomb without having him immediately engage my players?

2. How do I convey a sense of hopelessness? They're already running against the clock because his army of brood spawn is going to do their best to rush in after the group once they're inside. What else can I do to make him seem too powerful to defeat (without, y'know, making him too powerful to defeat)?

[Sipex]

I find the best thing to add tenseness is an obvious time limit. Not just "You have to do this quickly as the army could show up at any minute." as players tend to take that as "The army will show up when plot says it should."

Give them an actual time limit in turns or something. My climactic battle had the players rushing a 10 turn time limit to cross a large battle area with a bunch of enemies between them and the objective and having to solve a puzzle, mid battle, to disable the objective.

Other than that? It's really hard to make something seem impossible to kill without actually being impossible to kill.

You could run a 'run over' scenario where the players fight it in a sort of battle which is easy enough that they don't immediately wipe (so they feel they have a chance) yet they really don't have a chance. Then you sort of need some kind of deux ex machina to intervene to allow them to actually beat him, like a god or something.

[Anxe]

Earthquakes that restructure the tomb's geography and separate the characters within the tomb.

[Antonok]

Heres one that a DM pulled on my group.

DM: You stop at the top of a hill overlooking the entrance to the BBEGs lair. Theres a great open field in between where you stand and the entrance.

Player: Ok, I pull out my gem of true seeing. What do I see?

DM: The BBEGs army.

[Cheesy74]

Ideas I had while in class:

-Guarding the mountain itself with some pretty vicious homebrewed monsters to start with. Unique stuff that the players will remember.

-Have part of the ascent entail the players running alongside a mountain pass as weak but innumerable flying brood spawn swarm them (1d3 CR 4 brood spawn per round?). Their goal isn't to kill all of them - they're infinite in number - but to make it to the next cave without being knocked off the mountain (difficult, as one area of the pass shrinks to a tiny five-foot-wide platform).

-Give them various views of the army being pushed back and losing as they ascend.

Ideas for the climactic fight include the intervention (and destruction) of an avatar of Pelor to start things off and Llymic showing his power by weakening/damaging the Beacon of Pelor (an artifact staff my players grabbed that emits natural sunlight).

Other comments?

[Fiery Diamond]

Heres one that a DM pulled on my group.

DM: You stop at the top of a hill overlooking the entrance to the BBEGs lair. Theres a great open field in between where you stand and the entrance.

Player: Ok, I pull out my gem of true seeing. What do I see?

DM: The BBEGs army.

That gave me chills when I realized the implications of being able to invisibilify an entire army would have for the power of the BBEG.