Death By A Thousand Cuts

Damage Threshold 

Effect

15

You may add +2 to your attack rolls against this enemy.

25

You may add your proficiency bonus to the first damage roll against this enemy each turn. 

40

Your enemy takes 1d4 slashing damage for every 10ft it moves.  (This effect is cleansed if the enemy receives healing, if you deal another 40 damage it is triggered again.)

60

Your bonus to attacks against this enemy is upgraded from +2 to 1d4+2.

80

You can now add your proficiency bonus to all damage rolls against this creature. 

90

All creatures except for yourself who can see this creature receive a disadvantage on Wisdom Saving throws. 

100

This enemy takes 1d6 slashing damage each time it takes an action, bonus action or reaction. (This can be cleansed by the greater restoration spell, if you deal another 100 damage it is triggered again.)

115

This enemy receives resistance to fire damage due to being coated in blood. 

150

Your enemy receives disadvantage on Strength, Dexterity and Constitution saving throws. 

200

You receive advantage on all attacks against this enemy.

DM's are recommended to add social consequences to PC's who take this feat. While all combat is different, most of the time watching someone cut a guy to death is rather disconcerting and would cause some people to raise concern. My personal recommendation would be the PC receives disadvantage on persuasion and performance rolls on anyone who has witnessed the PC fight.