An ancient order of secluded Blood Hunters, Aabithar Siphoners have keeps in the most isolated places. For millennia they have been hoarding knowledge about their eniemies.  

Know Your Enemy

You can use an action to examine a dead or alive foe, whenever you fight another foe of the same species you get a +1 to hit, and you can also use Aabithar Siphon on them. 

Aabithar Siphon

You use a technique, that combines with your knowledge of the target you can siphon their life force. You can only do this on species you have examined with Know Your Enemy.  You gain 2d8(every 3 levels it increases by 1d8)+Proficiency Bonus hit points, while the enemy takes 2d8(every 3 levels it increases by 1d8)+Proficiency Bonus damage. This can be done as an action.  

Monster Specialist

Your knowledge becomes more enriched by your experience fighting monsters. You get advantage on Perception and Survival checks used to find the monster species you have already examined. 

Improved Examination

Your experience with examining is improved. You now get advantage on investigation, and your +1 from Know Your Enemy becomes a +2.

Quick Learner

Your constant learning lets you learn quicker. Whenever you use Know Your Enemy instead of using an action you can use a bonus action to examine the target.

Forgotten Knowledge

You learn so much about monsters that you can emulate some of their traits. Every long rest you can choose one of these traits (they last till the next long rest):

• Troll Regeneration: If you have at least 1 hit point on each of your turns you regenerate 5 hit points while in combat.
• Werewolf Might: Your strength goes up by 2  (this can surpass 20).
• Draconic Majesty: Your charisma goes up by 2 (this can surpass 20), and you gain a flight speed of 30ft.