Vow of Vengeance( Drow of the Underdark, p. 56)
Filled with hate, you have made an unspeakable vow to visit vengeance on those whom you believe wronged you and your people. Whether the affront is real or imagined, you settle for nothing less than the extermination of your enemy.
Unspeakable Vow (DrU) ,
Choose one creature type or subtype from those given on Table 3?14: Ranger Favored Enemies, PH 47. You gain a +2 profane bonus on damage rolls against these creatures, as well as a +4 profane bonus on rolls to confirm critical threats against them.Special
To fulfill your vow, you must kill at least one creature of the selected type each month. When facing these creatures, you cannot retreat and you must fight until you or all the creatures of your chosen foe are killed. You can take prisoners, but they must die within 1 week of capture.
If you intentionally break any portion of this vow, you immediately and irrevocably lose the benefit of this feat. You may not take another feat to replace it. If you break your vow as a result of magical compulsion, or otherwise unintentionally, you lose the benefit of this feat until you perform a suitable penance and receive an atonement spell.