Continuing to talk about whether, when, and how to ban divination and teleportation: in the previous post, I mentioned two major reasons why most of the popular solutions to limiting divination and teleportation don't work for me. The second reason is my feeling that such limitations need to be setting specific.
Consider teleportation. How does it work? One interpretation is that it makes two locations magically the same location, for an instant. This means that Detect Magic should work at the destination, immediately before the arrival. If that's so, an MU or enchanted creature with an ability to detect magic at that moment would have a warning, and could opt to cast Dispel Magic. It also means that a simple area of anti-magic (like Anti-Magic Shell) would prevent teleportation into an area.
But what if you don't see teleportation that way? What if you say it's instantaneous travel through the Astral Plane? In that case, Dispel Magic won't work because it's not cross-planar, but astral guardians to prevent entry may be an option. Instead of arriving at the expected destination, the teleporter arrives at a threshold area and must battle an astral creature before manifesting.
And here's another idea: what if the only physical law twisted by the Teleport spell is that of time? In other words, what if it allows instantaneous travel, but with the limit that the character must be able to physically make the journey? The character walks (or swims or even flies, if the character has that ability) to the destination, but does not experience the passage of time except as a fleeting impression. In that case, a room with a single locked entrance cannot be teleported into, and a failed Teleport would work a little differently: you wouldn't have a chance of failure, but would teleport to the closest location you could reach without unusual effort.
Or maybe Teleport is an instantaneous spirit-assisted physical journey: there must be a physical way into the location, but without a limit on your own physical capabilities -- and that might include routes you would be unaware of and perhaps unable to be aware of, but which are immediately obvious to the all-knowing spirits. So, you can teleport into that room with only one known entrance, but might not know how the spirits got you past that locked door. Is the lock operated by a switch? Is there a second, hidden entrance?
The point here is: how an NPC villain would plan against enemies teleporting into their stronghold depends on how Teleport works. It's better to pick a solution that fits your conception of your particular setting than to pick one that seems too meta-game, blocking a spell effect simply because it's useful to your design purposes.
Divination, likewise, depends on individual setting details. The Clairvoyance spell description states that metal will block the spell. That seems fine enough for a low-level spell, but I'm leery of using that as a general divination limitation. It certainly doesn't seem to fit any kind of ethereal or astral sight, for instance.
Another example: I consider clerical Dispel Magic to trump magical spells, but not vice-versa. That probably seems alien to most GMs, but it fits into a medieval feel, for me: shamans and priests work miracles with the aide of spirits, saints and gods, whereas magicians are manipulating esoteric forces within the world as it is. That, to me, suggests that a holy or unholy site might provide a barrier to divination and teleportation; not a fullproof ban, but a chance of spell failure based on the strength of the spirits defending that site. Further, the attempt to scry or teleport also prompts a reaction roll from the spirits, whether the spell succeeds or not. The offending magic-user might be plagued by bad luck or attacked by an angel or demon, depending on the importance of the destination.
That solution, as I said, seems alien to other GMs. It doesn't fit within the standard conception of how spells work. However, it fits my conception, plus it has the benefit of making stuff happen instead of forbidding stuff from happening, which is my general rule.
There's another limit I have that's specific to my preferences: as readers may have noticed, I prefer the old level limits -- no magic-user spells above 6, no clerical spells above 5. That's one reason why I skipped Vanish, Drawmij's Instant Summons, and Word of Recall: they are above the maximum level of preparable spells. That doesn't mean the spells don't exist, only that they can only be cast as rituals or from scrolls, and there is a chance of spell failure. That spell level boundary is unusual even among old-schoolers, so I have a spell limitation that doesn't fit other campaign settings.
(Another reason: the Instant Summons and Vanish, although technically teleportation, are different in intent from the standard, bringing an object to an MU or sending something away from an MU. Either one can be counteracted by Dispel Magic or an anti-magic field, so they aren't that difficult to block, and neither really fits the kind of "teleportation abuse" I see complained about in the forums. Word of Recall, on the other hand, seems awfully overpowered.)
There's also the fact that, although I'm discussing AD&D spell descriptions, I would actually be using much simpler spell descriptions with my own rule variants. For example, instead of the Teleport percentile error table, I'd probably roll 2d6 (extra dice for less clearly visualized destinations) and let doubles mean an error; if triple the value rolled is equal to or greater than Intelligence, the character teleports in low; otherwise, high. That's a minimum error chance of 1 in 6, riskier than Teleport in other people's campaigns. That alone should limit its use in hit-and-run attempts or find-the-quest-object "short-circuits", making Teleport primarily an escape tool or an emergency resupply tool. You teleport to a well-known, safe haven, not into a dangerous villain's stronghold.
Likewise, I think I'd totally rewrite Contact Other Plane as Contact Supernatural Being. Spellcaster specifies the relative power of the being intended for contact, rated in hit dice. This is added as a bonus to the chance to know the answer (hidden GM roll,) but is also a penalty on a save vs. insanity roll, with a further penalty equal to the number of questions and a bonus equal to the MU's hit dice. Oh, and there's a 1 in 6 chance that the spell attracts the personal attention of a supernatural being, not necessarily the one contacted, who manifests near or invisibly follows the caster.
That seems like a reasonable enough limitation on divination.