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?