Designing a dungeon for 9th level characters (3.5)

[flappeercraft]

So right now I'm preparing for the next session my group will be playing, and since I have the role of DM I'm making something different than what we have been doing lately. So I'm making a dungeon where a homebrew minor artifact is hidden in and they are going to go get it as sort of a sidequest to gain more power to be able to even touch the recurring evil guy they are facing who they have not met but they know exists and is way more powerful than them. I'm planning to make it really hard for them to fight through it so this would be more of a stealth mission rather than a kick down the door gameplay but I'm lacking ideas and inspiration on how to make it. Im planning to make it so it lasts for possibly 2 sessions (we do usually 2-4 hour sessions and they go sorta slow paced). Their group consists of a Fire gnome Monk 1/Sorcerer 4/Ur-Priest 1/Mystic theurge 3 (he qualified for mystic theurge with alternate source spell and he has troll blooded feat), a Human Sorcerer 9, an Aasimar Cleric of Erythnul 9, a Water orc mineral warrior Barbarian 1/Fighter 6 Warblade 1 and a new player will be joining us next session, he has not yet finished his character but he is going to make an Assassin character. Any suggestions on how should I do it and what should I add? All sources that are 3.5 and non updated 3.0 are allowed including Dragon Magazine but no PF backporting.

[John Longarrow]

Cleric + Melee tank normally =/= stealth.

If you are really committed to doing this, I'd suggest standard "Evil Temple" layout where there are obvious guards. If the PCs attack the guards then all hell breaks loose. If they bypass the guards (stealth, diplomacy, Successful use of save or suck) without a big fight, then they can start progressing further in.

Make sure they see something that screams "Don't go in swinging" so they know not to try melee to begin with. Course it also depends on the players. Some can't help themselves and always attack.

[NevinPL]

If you want to force players to be stealthy, the good, old: "I kill\destroy X, at the first sign of trouble" works nicely, provided they know that.
Replacing some monsters with hazards like molds, slimes, etc. can hamper the HnS attitudes too.
Or use "exploding monsters", like the infamous Gas Spore.

[Echch]

So you have just reached level 9? How about...
An adamantine clockwork horror to the face few Drowned? I think their Auras stack, so you don't want to melee them.
At CR 9, you could have them ambushed by 6 Hammerclaws (CR 4 each) if they aren't stealthy, which can each have the party make a fort save (even if only against DC 16) in a 30 feet cone against 1d4 rounds of stun. Sure, DC 16 isn't too much, but if you have to make it six times, there is a good chance you will fail once.

I think another good creature would be Cloaker Lords, CR 7 creatures that do most of the things Cloakers do, but also cast as a level 9 Wizard without components.
You could also give them 4 Ephemereal Swarms, but those critters need to get to the casters rather quick to use their distraction ability.

So all things considered... Maybe an abandoned pirate cave thingy?

Maybe a pair of Huge Storm Elementals as the mini-boss of the area (even if that is of a bit higher CR, which should be appropriate for a boss).