Avenger (3.5e Trait)
From D&D Wiki
You have a grudge that must be settled...
Benefit: When you gain this trait, you pick one individual creature as your target. You gain a +1 bonus to attack rolls to hit creatures of the same type and subtype as your target. You gain a +2 bonus to damage rolls against your target.
Drawback: You take a -1 penalty to attack rolls to hit creatures not of your target's type and subtype. If you can see your target, you take a -2 penalty to damage rolls against all creatures other than your target.
Special: If you target is killed, you remove this trait.
Roleplaying Ideas: Perhaps your father or sibling was killed by your selected individual, or perhaps it was a fiance that jilted you. It may even be something as simple as a rivalry of some kind, or perhaps a theft or game at which the other cheated.