The campaign that I am running is all homebrew. I use a lot of the rules/lore of the Forgotten Realms, but most of the things are my own creation. So basically, my characters found this massive portal to the Shadowfell where an army of demons was gathering. They decided to go into the Shadowfell and stealthily close the portal. The way it works in my world is that there is a sacred crystal sent from the gods called the Exmarz that has all sorts of power. It was recently discovered in the campaign that these crystals can open portals to the Shadowfell. And so the portal that they want to close is being powered by a crystal. 

The part that I could use some advice for is figuring out mechanically how my players would go about this mission. There are two large towers, one in the Shadowfell, and one connected by a bridge through the portal into the Material Plane. Each tower has a crystal that needs to be destroyed in order to close the portal. However, to get to the towers, the players need to sneak past the massive camp of Shadows. I tried making a map of the camp, but after trying to write encounters, realized that it would be impossible and boring to run. I’m not quite sure how I should run this, because if the whole point is for them to sneak past, then if they fail a check, then the whole army is there to attack them. In other words, having a simple ‘role for stealth’ encounter until they get to the towers wouldn’t work unless they all succeeded their stealth checks which is almost impossible. Does anyone have any suggestions on how I should run this encounter? Once they get to the towers, it’s fine, because then they are in an actual mapped building with a boss battle. It’s just getting there that’s the problem. I want it to seem like there are high stakes if they caught, but the problem is that if they do, then there’s no way they would all survive, and this encounter is not designed to kill them. 

I want it to be focused on stealth, but it’s a very large scale area where initiative doesn’t apply. If anyone has some rules or ways of running this that could make it hard, but not impossible, that would be very appreciated.

Do not do it all stealth.  Remember Lord of the Rings?   Yeah, the hobbits got caught.

Start with some stealth. 

Then switch to some social stuff as well.   "These are not the droids you are looking for.  These are an entirely different set of rebel droids trying to poison Darth Vader, not blow up his death star.  Whoops, did I say that part out loud?"

Plan on them being captured, not killed.  :D

Plan on them escaping.  ;(

Plan on them THEN going stealth.

Some secret travel underground in secret tunnnels the bad guys do not know about.  But maybe have some monsters to fight in them?

Get to the secret location that has.... McGuffin that allows players to teleport much close. 

Now go for stealth again.

Awesome feedback, thanks! I love the references and couldn’t agree more. I was thinking of the underground part, but didn’t think to have it be only a small portion of the adventure. Normally I don’t do last minute prep, so thanks for the quick advice :)

 You could try running it like a 4th edition "Skill Challenge." I think there is a blog post from a while back here that talks about how to do this. You can also Google the idea.