Things to defend against in a level 20 dungeon

[Darth_Versity]

So one of my players complained yesterday that we never do high level adventures sue to the slowing down of combat at higher levels. So I told him i'd run a one shot dungeon, with level 20's that they could just have fun with.

I've limited to books to:
PHB 1 & 2
DMG
MM 1
Complete series
Races series
Tome of Battle
Spell compendium
Magic Item Compendium

I'm going to run a Demon/Undead dungeon with Orcus (the fiendish codex CR 22 version) as the main villain. The problem now is making sure that no one will unleash some dirty magic combo that will end the fun on turn one. So what shenanigans should I be prepared for in this dungeon and what protections should I have in place to stop anyone taking out my challenges to quickly?

Also, if anyone has ideas for encounters or traps based on the theme, I'm all ears.

[Inevitability]

Contingencies are always good at preventing shenanigans, and something with the WBL of a CR 22 monster can afford to buy a few crafted ones.

[Elkad]

Unless your "dungeon" is actually a collection of tiny demi-planes arranged as hallways and rooms, expect the party to ignore your layout.
They'll teleport or phase through the walls or whatever.
Want a puzzle on a door, or a requirement they find the key? Make that door open to two different places, depending how it's opened.

[Jack_Simth]

So one of my players complained yesterday that we never do high level adventures sue to the slowing down of combat at higher levels. So I told him i'd run a one shot dungeon, with level 20's that they could just have fun with.

I've limited to books to:
PHB 1 & 2
DMG
MM 1
Complete series
Races series
Tome of Battle
Spell compendium
Magic Item Compendium

I'm going to run a Demon/Undead dungeon with Orcus (the fiendish codex CR 22 version) as the main villain. The problem now is making sure that no one will unleash some dirty magic combo that will end the fun on turn one. So what shenanigans should I be prepared for in this dungeon and what protections should I have in place to stop anyone taking out my challenges to quickly?

Also, if anyone has ideas for encounters or traps based on the theme, I'm all ears.

You have PHB 2 and Complete Arcane in there.

Get several Craft Contingent Spell for Greater Celerity and Favour of the Martyr on him (an artificer or warlock would be involved... warlock is thematically appropriate). Set them to go off in pairs, one pair at a time, whenever someone casts a spell at him and he didn't say "Shalkoombarna" in the previous round. Someone casts a spell at him, he gets a full round, THEN the spell goes off. And he's not dazed, and still gets his normal round when his turn comes up.

He has Astral Projection as an at-will. You're not facing him, you're facing his 5th generation projection... and at each step other than the first, he's got several quasits who's job it is to regularly Commune to find out whether or not he's stuck, and to trigger a trap destroying the projected step there (freeing him from whatever-it-was).

He has Symbol of Death as a 3/day. There should be a LOT of those around the dungeon.

He's Chaotic. More than that, he's one of the most powerful incarnations of chaos. The dungeon should reflect that. Doors lead to random places: Each one has six different places it could lead to (which are not necessarily reciprocal!), selected at random each time the door is opened.

[Eldariel]

Forbiddance across the board (potentially with a password the party could perhaps plausibly discover via Mind Rape or such). It stops dimensional movement, summoning and the like. Make sure to use portals and dimensions in creating it though - permanent portals could be excluded (particularly if the caster has Extraordinary Spell Aim [Complete Adventurer]). Perhaps having to use teleport and plane shift to progress at places. Perhaps Limbo-like subjective gravity could be a nice touch; achievable through permanent Planar Bubble: Limbo to the whole area. It can lead to all sorts of fun times. Makes the dungeon a bit more level appropriate (though the party can of course try and dispel it). Permanent Desecrates of course with Altars of Orcus.

[Elkad]

https://mindweaverpg.wordpress.com/2014/01/03/the-rotating-labyrinth/

Chaos! (not really, but it'll look that way to your players. Make them map it themselves.)

There are other maps based on non-Euclidian geometry, 4 & 5 dimensions (tesseracts and hyper-tesseracts, with or without subjective gravity), and other ways to annoy your players scattered about the web.

[Mr Adventurer]

Go through Dungeonscape and use everything in there at least once. Walls of force for walls and floors, zombie doors, walls of ooze. Acidborn Gravetouched Ghoul Pirhana Swarms in acid-filled rooms.

A door that requires teleporting to open: it's magically sealed and keyed to open only in the presence of a particular widget. The widget is formed of Limbo chaos at a shrine elsewhere, when removed from the shrine the widget only lasts for 1 round before dissipating.

Either prevent teleportation, or embrace that Outsiders do it at will. Progression from one area of the dungeon is not in geographical order, but because you need something from each area in order to progress to the next. Chambers thousands of miles apart, you need to raid one to find the location of the next in order to be able to teleport there.

Mirrors that can scry each other (they're in Complete Scoundrel), you have to go to one of the pair and use it to be able to see where to go next. At one point, there's a Mirror of Opposition instead.