SRD:Oathbow

Oathbow: Of elven make, this white +2 composite longbow (+2 Str bonus) whispers “Swift defeat to my enemies” in Elven when nocked and pulled. Once per day, if the firer swears aloud to slay her target (a free action), the bow’s whisper becomes the low shout “Swift death to those who have wronged me.” Against such a sworn enemy, the bow has a +5 enhancement bonus, and arrows launched from it deal an additional 2d6 points of damage (and x4 on a critical hit instead of the normal x3). However, the bow is treated as only a masterwork weapon against all foes other than the sworn enemy, and the wielder takes a –1 penalty on attack rolls with any weapon other than the ''oathbow. These bonuses and penalties last for seven days or until the sworn enemy is slain or destroyed by the wielder of the oathbow, ''whichever comes first.

The oathbow may only have one sworn enemy at a time. Once the wielder swears to slay a target, he cannot make a new oath until he has slain that target or seven days have passed. Even if the wielder slays the sworn enemy on the same day that he makes the oath, he cannot activate the oathbow’s special power again until 24 hours have passed from the time he made the oath.

Strong evocation; CL 15th; Craft Magical Arms and Armor, creator must be an elf; Price 25,600 gp; Cost 13,100 gp + 1,000 XP.