Knights of Sacrifice learned from the War of 10 Heavens that the willingness to suffer to win could turn the tide of battle.  With their innate ability Adversity, they increase their combat abilities the more damage they take.

Adversity

Beginning when you choose this archetype at 3rd level, when you take damage your effectiveness in combat increases.  Each time you take at least 10 damage in a single attack, you gain a +1 bonus to all weapon and unarmed strike damage.  If you fall to 0 hp, this bonus resets to 0.

Bloodletting

When you make an attack, you may take 1d6 damage, and you deal +2d6 necrotic damage.  The damage you take from this ability cannot be resisted.  If the damage would reduce you to less than 1 hit point, the ability fails.  At level 15, this ability deals +3d6 necrotic damage.

Resist Death

Starting at 7th level, you are resistant to necrotic damage.   In addition, you add a +1 bonus to your death saves.

Black Bane

As an action, you deal 3d6 damage to yourself.  You then select a point within 60 feet.  Necrotic energy explodes in a 10 foot radius dealing 6d8 necrotic damage.  Creatures in the area of effect  may make a constitution save (DC is 8 + proficiency bonus + your strength modifier) for half damage.  The damage you deal to yourself cannot be resisted, and if it would reduce you below 1 hp the ability fails and no damage occurs.

Improved Bloodletting strike

Starting at 15th level, bloodletting strike deals an additional 1d6 necrotic damage.

Blaze of Glory

At 18th level, you attain the pinnacle of sacrifice.  When you fall to 0 hp, an explosion of necrotic energy lashes out from you.  All creatures within 10 feet of you take 12d8 necrotic damage. Targets can make a strength save (dc 8+proficiency+str modifier) for half damage.  This ability can only be used once per long rest.