The character has taken great time and care to significantly adapt their behavior to the point of view and behavior of a particular prey/adversary (Chosen Prey. This must be some homogeneous group or species (or subgroup if not universal), individual, cult, brotherhood, a group intersection on a cultural and/or regional basis, etc. which has regimented, consistent, predictable behavior patterns that can be learned and countered through study, hunting, fighting and other interaction. It affects all checks to assess what they might do next such as plan attacks, as well as combat, and social situations. If the prey does not follow their normal behavior for some reason, all advantages are inapplicable.
Enmity is not simply a knowledge of an adversary, it is a personal antipathy due to some set of circumstances that placed them and the PC together in the past.
Rangers have their Favored Enemy replaced with this at no CP cost. A new chosen prey is taken at 6th and 14th levels.
Chosen Prey: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, monstrosities, oozes, plants, or undead. Alternatively, you can select two [non-character] races of humanoid (such as goblins or orks) as favored enemies. Prey choices must reflect the creatures you have encountered on your adventures or character background.
Focused Hostility: All your enmity is directed to a target Chosen Prey for 1 minute or until you lose Concentration. You must use your Concentration to gain the benefits, with each hit on the target doing an extra d4/2 damage. The extra damage increases at certain character levels: to 1d6 at 6th level and to 1d8 at 14th level. You can use this to mark a foe of the chosen type a number of times equal to your Proficiency Bonus, regaining all expended uses when you finish a Long Rest.
Study of Prey: You have advantage on Wisdom (Survival) checks to track your chosen prey, advantage on Charisma (Intimidate) checks to goad or fill with dread, as well as on Intelligence checks to recall information about them. You also learn the language of your chosen prey (at Basic spoken capability), if they speak one at all.
(ex: Lore: Military), as well as combat (Melee and Evade) and social (ex: Persuade) gain a +1 bonus. If the character possesses Natural Wisdom in a related subject (Lore proficiency > 4), they may purchase Natural Hunter (applied only to the target of Favored Prey). If they possess all three, they are considered a ‘Slayer’ of their prey: Lore: Military used against it gain 2 additional bonuses if successful; and they have Danger Sense against the prey target.