Is there a rule to create flawed magical items?

Question

For various reasons, I would need to emulate an effect from a famous magic item: the Helm of Brilliance

If a creature wearing the helm is damaged by magical fire (after the fire protection is taken into account) and fails an additional DC 15 Will save, the remaining gems on the helm overload and detonate. Remaining diamonds become prismatic sprays that each randomly target a creature within range (possibly the wearer), rubies become straight-line walls of fire extending outward in a random direction from the helm wearer, and fire opals become fireballs centered on the helm wearer. The opals and the helm itself are destroyed.

The Helm's "flaw" is a rather obvious one (though, player not knowing might have some surprise, basking in their fire resistance 30) - and while it was by design in the lore, it is clearly a disadvantage to the wearer.

I currently need to create a collar that will grant its wearer at least +2 Int and can not be taken off (except by me, so I will need to feature a Mage's Lock spell). The "downside" is that, if the command word is spoken in the vicinity of the collar, it will explode hard enough to take a troll head clean off (mandatory).

Obviously, this feature is a slight inconvenience to the wearer. When creating an item, the goal is generally to give advantages to the person or creature wearing it.

I was wondering if there was any rules to mitigate the cost of a magic item by including "flaws" (such as the one in the Helm of Brilliance), or if I have to resort to adding X fireballs centred on the item with a command word?

Answer 1

Create a cursed item

Flaws like that are handled as Cursed magic items, which had their rules expanded in the Black Markets Player Companion book:

Intentionally Crafting Cursed Items

Intentionally crafting cursed items requires the same item creation feats and skill checks as does crafting a normal item of that type, but in addition to such requirements, intentionally cursed items require bestow curse or major curse as a spell prerequisites.

Crafting cursed items is generally cheaper than creating fully functional items, depending on the type of curse involved, as detailed below. The table above indicates the price and spell prerequisites of some of the most common deliberately created cursed items.

Delusion: Cost is reduced by 90%.

Drawbacks and Requirements: Cost isn’t reduced for cosmetic drawbacks or requirements with no direct game effects. Cost may be reduced by 10% for minor drawbacks or requirements such as minimum skill ranks or worship of a specific deity; by 30% for harmful or costly drawbacks or requirements such as alignment change, ability damage, sacrificing wealth, or performing a quest to activate the item; or by 50% for severe drawbacks or requirements such as negative levels or sacrificing sentient creatures.

Intermittent Functioning: The cost of uncontrolled or unreliable items is reduced by 10%. The cost of dependent items, which function only in certain situations, is reduced by 30%.

Opposite Effect or Target: Cost is reduced by 50%.

I would say that a magical helm that normally allows you to cast Fireball, but has a chance of casting Fireball centered on the caster and likely hurting her, is certainly flawed to the point of being considered a curse similar to a Drawback, worth reducing the cost by 30%.