You are able to expend ki to strengthen your attacks.
On a hit with an unarmed attack, you may use 1 ki point to inflict an additional martial arts die in damage. If you have no ki left, you may still deal the additional damage die, but you take you 1d8 bludgeoning damage. Your maximum hit points are reduced by the amount of damage taken until you finish a long rest.
Once per long rest, you may use this feature to expend as many the ki you have left for that many additional damage dice, but you take 1d8 bludgeoning damage for every ki point used
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