Viper's FangConsumable 14
Source Pathfinder Core Rulebook
Price 850 gp
Weight negligible
Usage affixed to a weapon
Activate 
Envision
Trigger A creature within your reach uses a manipulate or move action, makes a ranged attack, or leaves a square during a moving action it's using
Requirements You are a master with the affixed weapon
When you activate this resin-strengthened viper skull, you make an Attack of Opportunity against the triggering creature.
If you have Attack of Opportunity, you can activate the viper's fang as a free action.
Traits
Anything that doesn't list another rarity trait (uncommon, rare, or unique) automatically has the common trait. This rarity indicates that an ability, item, or spell is available to all players who meet the prerequisites for it. A creature of this rarity is generally known and can be summoned with the appropriate summon spell.
ConsumableAn item with this trait can be used only once. Unless stated otherwise, it's destroyed after activation. Consumable items include alchemical items and magical consumables such as scrolls and talismans. When a character creates consumable items, they can make them in batches of four.
EvocationEffects and magic items with this trait are associated with the evocation school of magic, typically involving energy and elemental forces.
MagicalSomething with the magical trait is imbued with magical energies not tied to a specific tradition of magic. A magical item radiates a magic aura infused with its dominant school of magic. Some items or effects are closely tied to a particular tradition of magic. In these cases, the item has the arcane, divine, occult, or primal trait instead of the magical trait. Any of these traits indicate that the item is magical.
TalismanA talisman is a small object affixed to armor, a shield, or a weapon (called the affixed item). You must be wielding or wearing an item to activate a talisman attached to it. Once activated, a talisman burns out permanently.