Ok, imagine you are a council of at least moderately powerful religious divine casters. Not of the same deity nor even normally allied with each other. However you all agree that a particular item needs to be protected and kept out of circulation. Not destroyed because it may be needed someday, but definitely kept out of circulation. Not at any one deity's church because you don't really trust each other.
So you are all going to make a vault together. What kinds of permanent guards, traps, wards, etc... would you put in place.
You would want it to not be something that can be worn down with piles of mooks. So traps (both mechanical and magical) would have to be the kind that automatically reset.
Not just traps, because then it would just take a high perception and disable device to get through.
Haunts actually seem like a good choice since they naturally reset unless put to rest.
Golems are powerful guards, but don't heal and once eventually beaten they are destroyed.
Undead could be excellent guards and some/many of them do recover with time.
What other things or spells can you think of? I want different kinds of powers to be necessary to beat this thing. This will be late campaign. The party will be around level 12-15 (depending upon when they learn about it and decide to go for it).
I think I've got the final lock figured out, but the stuff leading up to that is still in the "to be determined" stage.
Oh, almost forgot. This council does not trust arcane casters and will probably not make much use of anything that absolutely requires an arcane caster to put in place. But something like a golem which could be passed to the control of the council then put in place would be fine.
There are ways for golems to repair themselves and even be brought back after being destroyed
A hallow effect
Undead(depending on the religion involved)
Outsiders(they don't age and would do it if you got them to agree to it)
Do note that with the power of miracle and UMD they can use all arcane defences
Let me see...in the webcomic Grrl Power is a vault under the control of a council of powerful groups where they store their artefacts. Partially because they cannot be destroyed, because destroying them would be a bad idea, or because they can do something extremely useful besides the bad stuff.
They have a massive door which can be opened by the combined touch of four council members. Inside is a massive golem with a special weakness aura, that turns (almost) everyone into easy prey. The comment of the golden heroine after beating the golem is hilarious :)
Then there is a ward preventing teleporation by normal means (and which the thieves bypass). Once inside the ceiling comes down and squishes everyone flat. There is also some kind of auto-shooting trap with something for everyone, from shrapnel to firebeams.
Around each item is an individual field that kills anyone alive touching it and blocks dead matter.
And of course, their best defense is that the location of the vault is known only to the council members, but not the public.
I did design a vault once for my high level 1E group. There was a nasty dungeon with deadly traps, rarely seen monsters, deadly illusions, and finally a treasury that was already plundered, with the skeleton of a robber remaining. The cunning gamers smelled a rat and checked the place again and found a secret door to a small treasury, which left them satisfied. The real vault was elsewhere, behind a secret door (which was an unusually shaped golem and could therefore not be picked or knocked open) in the entrance room, and completely clad in lead to foil most divinations. After all, would you expect the high level clerics to jump through all the hoops to make a deposit?
The whole thing is of course easy, if someone thinks to go through the priests instead of dungeoneering.
Does it need to be autonomous? Or could there be high-level NPC guardians?
Autonomous? Not necessarily, but remember the council members do not trust each other. Who does the NPC report to? Who might be allowed to secretly enter the vault without the others being aware?
Also, I intend for this to have been in the vault for hundreds if not thousands of years.
I would engineer the dungeon that houses the vault to be constantly mobile, either a floating sky island ...
That has definite possibilities. This will require some careful thought. (I was never a fan of the massive golems.)
...Getting access into the dungeon should be an adventure in and of itself.
Oh it definitely will be.
And adamantine golems have both fast healing 10 and
Indestructible (Ex) An adamantine golem is nearly impossible to destroy. Even if reduced below 0 hit points, its fast healing continues to restore hit points, though the golem is helpless unless above 0 hit points. It can only be permanently destroyed if reduced to negative hit points and then decapitated using an adamantine vorpal weapon—alternatively, miracle or wish can be used to slay it while it is at negative hit points.
So thieves would be likely to reduce it to negative hp and then move on quickly, leaving it to heal behind them. (If they're going to toss miracles or wishes around, they could use them to permanently disable just about any normally self-resetting trap you might set up.)
Obvious spells include forbiddance, antipathy/sympathy, and dimensional lock, but of course overcoming any passive defenses is only a matter of time, enough dispel magics, and enough disintegrates[l/i]. So you need active defenses.
Powerful divine casters might very well have powerful outsiders available to guard vaults - [i]greater planar ally can provide some really powerful creatures, as can plane shifting to your gods' home and negotiating for the services of some even more powerful outsiders. Golems of some sort are quite likely too, probably of several different types.
And then there's the sign on the door that says "By entering this vault, you agree that your true name shall be spoken in the ear of the Mighty Avatar of Justified Vengeance Against All Thieves, and you agree that if you steal anything from this place your soul will be bound to serve him as a hunter for a million years."
All the "magic circle against ____" spells combined, surrounding the vault, with I believe stone shape to cover the silver used in the spell to prevent any true neutral character from interfering with it. Also using metamagic to increase the size of the circles to make it fully surround the vault.
This should keep most people out.