gizmo33 said:
So how would you create a "city of mystery" with this technique? Seems to me like you're changing the "City of Mystery" to "Unknown City". I see those as being different.
Well, it is A technique, not THE technique. To make a city of mystery, you need to do a few things. First off, make knowledge of it just that...mysterious. Have NPCs who provide information be just plain wrong about details. Give it special properties and legends about those properties. Take the Mournlands from the Eberron setting, for example. With the common knowledge that healing doesn't work there, no one goes there. What is the city that the Lord of Blades rules from like? Who knows? Perhaps the city only appears during certain times, fading in and out with the moon. Perhaps its can only be entered when the tides go out. Perhaps the city suffers from a horrible plague, such as the Vanishing from the Shackled City adventure path. Note that many 3.5 divination-type spells can be resisted or fooled fairly easily. Find the Path has a short duration AND only works for the caster AND doesn't take into account the actions of enemies or guardians.
Here's a few ideas:
1)
Time does NOT have to be on their side. Give a wizard 24 hours and he can solve any problem. Read about Piratecat's party using Earthquake to collapse a dungeon to get to the villain inside, for example.

BUT, if you they have to solve the problem in the next hour, they need to improvise. If they have to enter the mysterious city within the next hour because it only appears on the white mountain when the moon is in full, and will have to wait another night, month or more to enter again, you understand why the city is mysterious.
2)
Make the goal multi-layered. Most high-level spellcasters DO NOT memorize/prepare or otherwise HAVE lots of redundant utility spells. Teleporting twice is easy. Teleporting an entire group three times or more....suddenly they become a little more miserly about those spells. The city is mysterious because it's trapped BENEATH the frozen lake. The ice is far too thick and deep to penetrate...but there IS a magic item that could melt their way through...200 miles to the West in a temple of Pelor.
3)
Demi-planes are your Friends. Old school Greyhawk was loaded with areas that had become alternate pockets of reality on the Prime...in essence become another plane in a limited geographic area. Eberron has this idea to, with certain places (such as Sharn) being closely aspected to certain other planes. There's no reason you can't use this to your advantage. Again, using the Mournlands as an example, the rules are different there. The Mysterious City could create new limitations for those within it...that offers both benefits and penalties. Make sure that the environment is memorable, without seeming like it was simply made to take the PCs powers away. Powerful heroes face powerful challenges. Imagine that our sub-ice city is negatively-aspected and limits divination magic to 100' per level while they're in the city, and causes divintation spells from outside the city to fail if the target makes their save. Just make sure such things work fairly against the bad guys, too.
4)
Prepare less in advance stat-wise, and more in advance story-wise. PC uses (or at least used to use) MS Excel for his maps. Detailed? No....but simple and easy to create and use. I tend to create simple outlines for high-level adventures, using mostly bulletpoints to remind me of big info. I usually have a few pages of stat-blocks and not much more.
As you've sussed out, high-level D&D is a much different animal in terms of adventure design. Plan for fewer, but tougher, encounters. Don't handwave some restrictions unless you want to; Teleport, for example, has some important weight restrictions. Sometimes, you can create a challenge that you're not sure how the PCs will solve...depending on the group, it might be better to see what they come up with, and if it sounds good, go with that. Let your players surprise you with their ingenuity.
gizmo33 said:
It's hard to create a sense of drama about a villain (or city, or item) that no one knows exists - especially in the way it's done in many "typical" adventure stories. Sure, there might have been a way to write LotR so that no one ever knew about Sauron or the Ring until the last minute, but I have the feeling that this technique would restrict you to a narrow set of adventure themes all resembling the X-Files. "Here's a box, take it to the place marked on this map. I can't tell you why or who I am. Good luck."
Note that I didn't say they didn't have a name...just that the players don't know who they are, in specific. Who is the Laughing Man? He's the head of the theives' guild...but who IS he? And who is the TRUE master of the guild...the one who pulls HIS strings? Knowing that the Nightsinger guild is coming for you doesn't help if you know nothing of their master, other than his title. Many villians also maintain mutliple names and use powerful enchantments to protect themselves. Note the important footnote for Scrying is "
You must have some sort of connection to a creature you have no knowledge of." Knowing OF the Laughing Man doesn't mean you can scry him.
Any high level wizard who doesn't use mind-blank most of the day is rarely worth the name. Don't forget the utility of Lead shielding, either. One villain I had maintained a Contingency spell that cast an illusion on himself whenever he was scried, for example. High-level evil clerics employ weaker clerics to do divinations of their own, in service of their master. It's not unreasonable to assume that High-Priest Morlang demands that his underlings each cast some spells such as commune on a routine basis. "
My master! Commune tells me that you will be attacked three days from now! It also tells me if you travel to the Stygian depths, your enemies will not follow you."
Using low-level dopplegangers is a common trick, as well. It only takes an alter-self spell to APPEAR like the Grand Foozle, without actually BEING him. Powerful bad guys also use lackeys. My players have indirectly negotiated with more than one ultra-powerful foe through an intermidiary...usually an incredibly weak one. When a 3rd-level expert or 4th-level bard comes up and says "The Laughing Man sends you a warning.", and the message was given to him by someone with a false name from another man who got a note from a courier....well, you get the idea.
Other ideas to consider is the classic 'Golden Child' macguffin. Sure, your players are 15th level...but Prince Surrabar is only a 1st level aristocrat...and they can't just travel around with him willy-nilly without getting him killed. Travelling about the city on foot is just the thing to do....THIS TIME.
Also consider that maybe the players can do other things there. Instead of having the city hard to reach, have it hard to remain in. Perhaps the head of the temple tells the paladin he needs to make the Dead City livable once more for the faithful, no matter the cost. Perhaps they spend their days exploring the city slowly, to make it safe, not for them, but for those who follow after. And so on.