Oath Of Retribution

An oath of Retribution is a dire promise to right a wrong, to take vengeance on an oppressor or the sworn destruction of any entity allying themselves with the object of your holy oath. Whether sworn in deafening silence and ceremony before the altar of Hoar, or rather promised to your comrade's family as they are lowered into the ground, or even as a final act of retribution as your body lies broken at the bottom of a ravine; the oath of Retribution is a solemn duty to bring righteous vengeance upon the guilty, the evil and those who have done you irreparable harm.

Tenants of retribution

An eye for an eye, a tooth for a tooth: A wrong dealt to me is a debt repayable only in blood. 

No innocent observer: Those who stood by passively are as guilty by association, and shall meet the same fate as my oppressor.

A gallon of blood for a pound of flesh: no cost is too high, no injury too great to stay my retribution.

Level 3: Channel divinity

Coup De Grace: as a bonus action change your d20 to a d24 for the rest of your turn. On a roll of 20 or higher you critically strike. On a Critical strike roll a d4, Multiply the bonus damage from the critical strike by the number rolled. After you critically strike for the first time the dice changed back into a d20.

Heart scales: As a bonus action put the soul of an enemy for trial against your own for one minute. Every time they deal damage to you, or impart a negative effect upon you roll a d20. On a 20 heal for all damage dealt and deal the same value to them as radiant damage, or impart the negative effect back onto them untill the end of your next turn. On a 1 the damage critically strikes against you. 

Aura of Retaliation

When you or an ally receives damage from an attack your Crit range is extended to 19 on your first attack against your attacker.

Bloodthirst

When you hit an attack regain hitpoints equal to your proficiency bonus. If this is your second attack this turn double the given healing. On a critical strike triple the given healing. If you have your total hitpoints deal damage equal to the value given as healing to the target instead.

Penance

Once per dawn or dusk (your choice when you gain this feature) as a free action assume the form of an avenging angel for 1d6+4 rounds While in this form any attacker takes half the damage it deals as radiant or fire damage (your choice when you gain this feature). Additionally you may make an additional attack for every Attack you land. Finally while in this form the effects of your other features from this subclass are doubled. You cannot die by any means untill this form ends, where you take the cumulative total of all damage and effects dealt to you while in this form.