With your time spent as a monk, you have found that inflicting pain is everything in a fight. Even if this means you must experience pain of your own. You have learned how to gather the power known as ki more effectively by using your own life force. You may get a few scratches, but that is a small cost for greater power. (Btw this is my first homebrew subclass so let me know if there is something wrong with it so I can improve it.)

Empowering Pain

Starting at 3rd level you have the choice to restore ki points as a bonus action. Using this ability will replenish 1d4/2 (round down) ki point(s) but will take damage equal to that dice roll multiplied by 2. You can also choose to just take 4 damage and replenish a ki point. You can not go over the maximum Ki you have. If you have full ki you can not use this feature. If you gain 2 ki and only have enough space for 1 you get back 4 health.

Bloody Strike

Starting at 6th level you can use either a bonus or regular action to spend a ki point to add an additional 1d4+player level-3 radiant damage on your next successful attack. But by using this ability you take damage equal to your player level. 

Lively Book Mark

Starting on 11th level you have the ability that once per short rest as an action, lets you spend 2 Ki point to set a sort of book mark in time for you to go back to. Once used, nothing happens until 1 to 3 rounds later (you get to choose). It gives you all the things like health and buffs from the turn you used it on. If you have to make death saves in the time frame before you go back, then instead of making death saves you just stay down until you a returned to your previous point.

Share Pain

once per long rest at the cost of 3 ki points, you can make an unarmed strike that will bring the health of an opponent down by the number of your max hp - the health you currently have. So long as you are in range you will not need to roll to hit and the attack will instantly land unless the target makes a reaction that says otherwise. This will cost an action on this turn plus an action on your next turn.