What are the mechanics for special-trigger magic items?

Question

I'm running a campaign in the Eberron campaign setting using the Pathfinder rule system.

I'm confused about the mechanisms for crafting wondrous magic items that activate upon certain conditions being met.

I'm thinking of implementing the "if-recipes" of IFTTT for magical item creation: "If ____, then do _____."

Normally, Pathfinder allows for these magical item activation methods (source):

• Spell completion
• Spell trigger
• Command word
• Use activated

Unless I'm misunderstanding the breadth of the above activation methods, there are conceptually lots of other ways to activate spells. Magical traps, for example, have several other trigger mechanisms.

Here are some example triggering/activation mechanisms I am specifically thinking of:

1. When [specified target]'s HP is below a certain amount, do ____.
2. When the item is a given distance away from [specified target], do ____.
3. On/off switch (as if to build a magical flashlight): when "on", do ____.
4. If the item comes in contact with [specified target], do ____.
5. If a [given alignment] creature comes within [distance], do ____.
6. If there is [certain level of] ambient lighting, do ____.
7. If the item is pointing at [specified target], do ____.
8. If [specified target] becomes [specified status], do ____.

These kinds of triggers could potentially be used for the following example devices:

1. Automatic potion-imbibing devices when its owner is low on HP.
2. Self-destruct device if a person forgets some important item somewhere
3. Magical flashlight
4. Booby-trapped bags of holding
5. Device that casts True Sight on owner when near evil-aligned characters, or perhaps Hide Alignment near good-aligned characters
6. A magical staff that casts Light at night and upon entering a dark room
7. A crossbow that automatically fires a bolt when [given target] is in its cross hairs
8. Necklace that casts a buff when its owner falls prone

(Never mind the fact that some of these triggers/conditions could be used to break the game!)

There are plenty of great guides for crafting traps and magical items in Pathfinder, but there doesn't seem to be one for the kinds of cause-and-effect crafting I'm talking about above.

Are there any general rules/guides/etc. for crafting such wondrous magical items? In particular, I'm looking for some mechanics for doing it, some way of estimating the Craft DC, and finally how to assign the price associated with it?

(If not, are there any rules of thumb or best-practices?)

Obviously, the mechanics, pricing and DC would vary widely with particular items....

Answer 1

Proving a negative is impossible, of course, but I'm fairly confident that there are no general guidelines for creating magical items with triggers other than the main four from the question:

• Spell completion
• Spell trigger
• Command word
• Use activated

... plus continuous items.

However, the Contingency spell would probably be useful for the "when X happens, do Y" items.

For pricing such items, the only real rule is to find an existing item of similar power and price your item similarly. The Item Creation Rules have guidelines, and there's some evidence that an item that requires the "right" slot to duplicate a feat costs around 5k (eg., the Gloves of Arrow Snatching, which allow use of the Snatch Arrow feat twice per day), but the rule is to price it based on similarly-powerful items.

I would specifically recommend asking whether a generic character would buy the new item instead of a +X weapon, armor/shield, Stat item, or cloak of resistance: if a generic character is definitely going to buy the item instead of a +2 shield, the price should be higher than the 4k a +2 shield would cost; if it's a toss-up between the new item and a +1 weapon, a price of around 2k is probably about right. This is difficult, especially for new GMs or items that are highly situational (eg., the "self-destruct if a person is too far away from an item" item) or setting-dependent (eg., the auto-healer if divine magic is rare in "this" world).

In my experience, it's generally been best to err slightly on the side of making the item too expensive, then adding some extra treasure if need be.

Remember, too, that "Activating a magic item is a standard action unless the item description indicates otherwise." (Using Magic Items) The rules don't preclude magic items that can be activated as move/swift/free actions, they just don't provide much in the way of guidelines for making them.

That said, it's entirely possible to build most of the items in the question's list using just the Big 5 triggers:

• Self-destruct device if a person forgets some important item somewhere
  • continuous item using Alarm and/or Scrying as a base requirement, plus a sufficiently powerful boom-ey spell
• Magical flashlight
  • use activated or command word item based on several of the Evocation (light) spells, eg., light
• Booby-trapped bags of holding
  • again, adding Alarm to the base requirements could serve to trigger the booby trap, as could simply calling it a variant Bag of Devouring
• Device that casts True Sight on owner when near evil-aligned characters, or perhaps Hide Alignment near good-aligned characters
  • again, a continuous item with Alarm and the appropriate Detect spell (eg., Detect Evil)
• A crossbow that automatically fires a bolt when [given target] is in its cross hairs
  • this is a reasonable interpretation for how True Strike works; an item that allows the wielder to use True Strike with a single attack from a weapon could be said to do this; similarly, Haste grants an extra attack in some situations...
• Necklace that casts a buff when its owner falls prone
  • a constant item with Contingency as part of the creation process could probably do this
• Automatic potion-imbibing devices when its owner is low on HP.
  • again, Contingency could probably be used in creating this