You lost a fight once to a horrible foe.  You vowed to learn from the experience - how it fought and how to kill it.  You've invested a short education to just that pursuit.  Choose a hated foe from - Aberrations, fiends, constructs, undead, beasts, or fey - and gain the following benefits when facing them

* You have advantage on (wisdom)Perception and (Intelligence)Investigation checks made to track your chosen foe
* You know how and where to strike. 18's and 19's now count as critical hits
* You know how they move, and are considered "Dodging" between your turns after taking a turn near this enemy without attacking it
* Your hatred fuels your attacks. Twice per long rest, you may add 2 times the creatures CR (minimum 2) to your attacks against your foe type for your next 3 turns