How does one trap a magic item?

Question

As a GM, I would like to introduce a "trapped" magic item in one of my next adventures. The idea is that interacting with the item (detect magic, most probably, or touching it for a moment) would trigger the trap, a magically induced unconsciousness.

Is this reproducible using the rules? Would this be a Curse or a Magic Trap? I'm thinking of the Sleep spell for the kind of effect, but I'm unsure which direction to take.

Admittedly, I initially thought of simply forcing this effect and giving a Hero Point for the trouble (think Fate compels or Mutants & Masterminds complications), but now I realize it wouldn't break the story if the targeted PC had a chance of noticing the trap and avoiding it. Having rules to know the DCs would thus be helpful.

I'd also like this trap to go off once only so the (genuinely magical) item remains as a possible reward.

A few details of the scene as I envision it:

• We are still a low level (4th) and the baddies should probably be limited in what they can do here. I'm thinking level 6 stuff is still fair, but beyond it strains believability
  • Limitations on the effect may justify a lower level. To take an answer as an example, a "Lesser Symbol of Sleep" could be a thing with limitations that fit my needs such as "single target, 5ft range or touch".
• I envision the limitations forcing this trap to be delivered in person and making sure the target interacts with it. The target being a magic user, I thought the guise of needing the item magically identified would be the trick. Use Detect Magic, and if you don't beat the regular DC +10 (ala Curse), boom, you don't notice the magic trap and it affects you. Alternatively, a touch-trigger with a Will save to resist the effects sounds good too.
  • Mostly, this is to fit my vision of the scene, but also to avoid total paranoia from then on. I don't want the party to think everything around them can explode at any moment, without any way to track down the culprits, for example :)

Answer 1

There exists Symbol of Sleep Which is a spell that does exactly what you want. I put the description here, and left only the most important bits in.

This spell allows you to scribe a potent rune of power upon a surface. All creatures of 10 HD or less within 60 feet of the symbol of sleep fall into a catatonic slumber for 3d6×10 minutes. Unlike with the sleep spell, sleeping creatures cannot be awakened by nonmagical means before this time expires.

Until it is triggered, the symbol of sleep is inactive (though visible and legible at a distance of 60 feet). To be effective, a symbol of sleep must always be placed in plain sight and in a prominent location. Covering or hiding the rune renders the symbol of sleep ineffective, unless a creature removes the covering, in which case the symbol of sleep works normally.

As a default, a symbol of sleep is triggered whenever a creature does one or more of the following, as you select: looks at the rune; reads the rune; touches the rune; passes over the rune; or passes through a portal bearing the rune. Regardless of the trigger method or methods chosen, a creature more than 60 feet from a symbol of sleep can’t trigger it (even if it meets one or more of the triggering conditions, such as reading the rune). Once the spell is cast, a symbol of sleep’s triggering conditions cannot be changed.

...

You can also set special triggering limitations of your own. These can be as simple or elaborate as you desire. Special conditions for triggering a symbol of sleep can be based on a creature’s name, identity, or alignment, but otherwise must be based on observable actions or qualities. Intangibles such as level, class, Hit Dice, and hit points don’t qualify.

When scribing a symbol of sleep, you can specify a password or phrase that prevents a creature using it from triggering the effect. Anyone using the password remains immune to that particular rune’s effects so long as the creature remains within 60 feet of the rune. If the creature leaves the radius and returns later, it must use the password again.

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Note: Magic traps such as symbol of sleep are hard to detect and disable. A rogue (only) can use the Search skill to find a symbol of sleep and Disable Device to thwart it. The DC in each case is 25 + spell level, or 30 for symbol of sleep.

Material Component Mercury and phosphorus, plus powdered diamond and opal with a total value of at least 1,000 gp.

The symbol can be placed on the item, or on a container it is in, or a surface it is near, or simply as a defensive measure to get to the item. The symbol disappears when the spell is triggered. So it is a one-time-use only (unless you use a Permanency spell)

Answer 2

A Custom Solution

It seems you specifically know what you want - so why not create that?

As a Magic Item (you want the PCs to be able to use this later)

Ring of Eternal Slumber

Aura moderate enchantment; CL 6th

Slot none; Weight 1/2 lb.

DESCRIPTION

This device appears to be a ring of mind shielding, and requires a DC 25 Spellcraft check, as well as the use of detect magic or arcane sight to tell any different. However, if a creature dons the ring, or examines it closely and fails to tell it's true nature, that creature must make a DC 19 Will Save or fall into a deep slumber from which they cannot be woken, except by a Remove Curse spell or the bard spell Reveille. If not magically awoken, they awake on their own in 1d10+6 days. Although in the shape of a ring, this is not a true magical ring, and does not need to be worn on the finger to be effective.

CREATION

Craft Wondrous Item, Deep Slumbers; Cost 2,750gp.

As A Magic Trap (fires once, then it's gone)

Cursed Rune of Sleep CR 6

Type spell; Perception DC 28; Disable Device DC 28

EFFECTS

Trigger touch, magic (see effect); Duration 10 days; Reset none

Effect Upon being handled by someone with magical abilities, the handler must make a DC 19 Will Save or fall asleep for 10 days. He cannot be woken by any means other than a Remove Curse spell that makes a DC 16 CL check to remove the curse.

Answer 3

Shorter Answer

It should be a trap because it's a externally placed (I assume) one-shot effect on an otherwise permanent magic item, but this doesn't really matter because the base DC ends up the same regardless of whether the item is built as a trap or as a cursed magic item due to the nature of the relevant formulae (specifically, they are actually the same formula, just worded differently):

10 Base +1 Spell level + 0 Int required = 11 if built by a wizard for the Cursed item

and

10 Base + (1 Spell level * 1.5) = 11.5 for the Magic Trap, which rounds down to 11

A DC 11 Will save is not going to be reliably failed by even an unprepared first level caster built by a first-time player, so you will probably want to raise this, which you (and the villains constructing the item) can do easily and cheaply, though the sets of methods usable for each design path differ. Using Heighten Spell Metamagic Gems is probably the simplest and most universal way to do this (but one of the more expensive).

Longer Answer / Design Philosophy Discussion:

Cursed Magic Items don't hurt random people. Cursed Items only hurt people who try to use them, or victims of clever people exploiting the specific nature of a Cursed Item by tricking it into thinking the victim is trying to use it. Necklaces of Strangulation and Poisonous Cloaks and such kill you when you put them on, presumably trying to gain some perceived beneficial magical effect. -2 Cursed Swords and Planar Invasion Shields and such abandon you when you most need them and bring with their abandonment problems worse than the ones you were presumably already facing. The biggest reason for having Cursed Items is to keep the air of mystery and danger around newly found magic items and force players to think I don't know what that is yet. Is it worth the risk to try it on? every time they encounter something new in the dungeon and can't just cart the item back to town for identification.

Traps hurt random people. Sure, the trap designer probably has specific people (like thieves) in mind when designing the trap, but traps are triggered more or less by existing in the wrong place at the wrong time. Their effects can be avoided, perhaps even by being the right kind of person, but, because they are triggered by exploration rather than exploitation, they are inherently less discriminate than Cursed Items are. Trapped treasure chests are an exception and function much like very minor cursed items.

Cursed Items are powerful. Many of them don't even allow saves, and have effects that are almost irreproducible. If you compare any of the specific cursed items in pathfinder to the specific non-cursed items of the same kind and caster level, you will find that the cursed items are usually much more potent.

Traps are weak. If you compare any trap of a given CR to a monster of the same level, the monster is usually the superior threat unless the party is particularly incautious. Traps are expected to be unexpected, and so are weakened to balance out the fact that they basically always have surprise if they manage to trigger.

The effect you want is a trap, not a curse. Rules for building such a device do in fact exist, and you can find them here.