Chosen Foe( Drow of the Underdark, p. 47)
[Fighter Bonus Feat, General]
At the expense of attention to other distractions, you focus on a single opponent to give you a decided advantage while fighting against him.
Once during your turn as a free action, you can designate a single opponent. You gain a +1 insight bonus on attack rolls made against that opponent and a +1 insight bonus to AC against attacks made by that opponent. In exchange, you take a —2 penalty on attack rolls against other creatures and a —2 penalty to AC against attacks made by other creatures. The bonus and penalty last until the start of your next turn.
A fighter can select Chosen Foe as a fighter bonus feat