This subclass is based off the job from Final Fantasy.

A knight who has trained to become the strongest, and steeping themselves in dark powers. A dark knight doesn't fight for good, but usually their own desires. They have abilities that outrank most other fighters, but lack sturdiness. Their darkness also makes them an opposing force to paladins, both being knights of their sides.

Sacrifice for Power

Starting when you pick this subclass, you give some of your own life to gain increases power. When you make health rolls upon leveling up, you subtract 1 from those rolls, but they can't go lower than 1.

You can also activate an ability, known as Soul Eater, as an action. At the start of each of your turns, you will take 1d4 necrotic damage. While this is happening, you have +1 to your AC, and you get critical hits when you roll a 19 or 20 on the die. This effect and damage will continue until it has been 1 minute, or until you fall unconscious.

Sanguine Sword

Beginning at 7th level, you can use Sanguine Sword. You can make a melee weapon attack against a creature within range, and on a hit, you can activate this. The target will take 4d6 extra necrotic damage, and you will regain hit points equal to the necrotic damage dealt. This can be done as many times as your intelligence modifier (minimum  of 1), per long rest.

This necrotic damage increases to 5d6 at 11th level, and 6d6 at 17th level.

Additional Fighting Style

Starting at 10th level, you can choose another fighting style option, which must be different from the one you originally chose. This is due  to dark knights being masters of combat, and excellent fighters.

Archery

You gain a +2 bonus to attack rolls you make with ranged weapons.

Defense

While you are wearing armor, you gain a +1 bonus to AC.

Dueling

When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon.

Great Weapon Fighting

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the two-handed or versatile property for you to gain this benefit.

Protection

When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. You must be wielding a shield.

Two-Weapon Fighting

When you engage in two-weapon fighting, you can add your ability modifier to the damage of the second attack.

Sacrifice Improvement

Beginning at 15th level, you have perfected the fundamentals of being a dark knight. When you activate Soul Eater, you now get a +2 to damage rolls, and a +1 on rolls to hit. You may also choose to take necrotic damage equal to your proficiency bonus to reroll damage rolls, but you can't reroll the same roll twice with this.

Unholy Sacrifice

At 18th level, you have now gotten the most powerful ability at a dark knight's disposal. As an action, you can target all enemy creatures within a 30 feet radius. Those creatures must make a charisma saving throw against 8 + your intelligence modifier + your proficiency bonus. On a failed save, you will take 30 necrotic damage, which cannot be healed until you take a long rest. The targets will take 20 + 10d4 necrotic damage as well. They take half as much on a successful save. The necrotic damage the enemies have taken can not be healed by spells of 5th level or below.