Martyr's Vow

When you roll initiative, you can choose a number of creatures equal to your proficiency bonus.

Whenever one of the chosen creatures takes damage, that damage is reduced by an amount equal to your Constitution modifier, and you take the same amount instead.

At level 5: Your hit point maximum increases by an amount equal to 10 × your Proficiency Bonus.

At Level 11: The reduction equals half your Constitution score, and you take that amount of damage.

At Level 17: The reduction equals your full Constitution score, but you take half of the reduced damage instead.