Death Knight

Death Knights enhance their fighting prowess with the ability to cast various spells through a pact with a dark entity. They achieve great power in this manner, but they also make great sacrificies as their humanity slowly fades away, with most of them eventually turn into undead monstruosities with and unquenchable thirst for the life force of all living things. This gradual loss of emotions is inevitable, but those death knights with enough willpower may be able to avoid complete loss of mental control; although they will eventually become detached from all human emotion and openly show disdain for mundane problems.

These powerful individuals tend to have their own goals and long-term objectives, and some even take on promises or oaths they made while they were still alive, becoming secret guardians of certain cities, eternal sentinels of a tomb or sacred place, or even as the secret weapon of war of a corrupt king. The exact nature of their goal varies from one death knight to another, and they may not seem obvious at first since these unholy warriors are not concerned with the passage of time anymore.

Spellcasting

As you reach 3rd level, your patron grants you the ability to cast spells. You may choose 2 cantrips from the warlock spell list, and a 3rd one of your choice at 10th level. In addition, you gain the cantrip of Chill Touch if you don't already know it, and does not count towards you number of known cantrips.

Your state both for spellcasting and cantrips is Wisdom.

Your spellcasting slots are deterimined by you death knight level, as shown in the table. You gain access to the following spells as you level up:

Level 5: False life, Ray of Sickness, Inflict Wounds

Level 7: Deafness/Blindness, Ray of enfeeblement, Gentle repose

Level 9: Animate dead, Feign death, Speak with the dead, Bestow curse, Revivify

Level 13: Blight, Contagion (will be cast using a 4th spell slot + a soul)

Spell save DC: 8 + Wisdom modifier + Proficiency bonus

Spell attack modifier: Proficiency bonus + Wisdom modifier

Death's cunning

Your mind is no longer bind by the same rules that chain those of a mortal. You gain proficiency in Arcana and Wisdom saving throws.

Nonetheless, your frame weakens as you get accustomed to your curse. You lose you Constitution saving throw advantage, as befits a fighter.

Life leech

At level 3, your dark patron grants you the ability to find nourishment in the life force of others. Whenever you grapple and opponent, you can use your bonus action to inflict 1d4+ your constitution modifier + your proficiency bonus of necrotic damage to the target, and heal for the same amount of damage dealt (or twice that amount if you are under 25% of your maximum health).

The damage of this feature increases to 1d6 at level 8, 1d8 at level 13, 1d10 at level 17 and 1d12 at level 20.

If the target is below 10% health, the dark hunger you hold within you becomes ravenous . The damage from this features is tripled against said targets.

If the target dies due to this effect, you instantly regain health equal to five times the creature's constitution modifier (minimum of 5).

Undead body

Beginning at 3rd level, you surrender your life to your patron in exchange for power. Your skin becomes completely white, and your eyes glow with a faint spectral light, akin to the color of your magic.

You gain resistance to necrotic damage, and from now on you are classified as an undead creature, so you are subjected to the same advantages and vulnerabilities as all other undead abominations. You stop aging, and you do not need to eat, drink, breath or sleep.

You no longer benefit from health potions, and any healing you received via spells other than your own is halved.

Reap the sowed

Beginning at level 7, when you use your action to cast a cantrip, you may use your bonus action to perform a single weapon attack.

Devourer of souls

Starting from level 7, every time you kill a target with the Life Leech class feature, you reap their soul from their body storing it within yourself to empower your powers.

At any given time, you may use any number of souls as a bonus action to recover a spell slot. You spend 1 soul per level of the spell slot. These souls inmediately are subtracted from your total soul count; but they will be returned if the regained spell slot is not spent before the next short rest.

When you roll the damage of an attack, you may also spend a soul to increase the damage by 1d20 of necrotic damage per soul spent. This souls are lost forever, and cannot be gained by any means other than reaping more.

In addition to it, for every 20 creature souls or for every 10 human souls, your maximum hitpoints increase by 1 per fighter level. If you spend souls to perfom any action, you might lose these bonuses.

Profane Flesh

As you become stronger, the dark energies within you take hold of your body granting you unnatural resilience and might. Beginning at level 10, you gain advantage in all strenght and  saving throws and ability checks, and also regain your previous proficiency in Constitution saving throws since your body already embraced your condition.

Your dead flesh also becomes hardened, granting you resistance to all slashing, piercing and bludgeoning damage from non-magical weapons. This also changes your appearance, making black veins show under your skin and increasing the spectral glow of your eyes. You gain proficiency in all intimidation checks and gain inmunity to necrotic damage.

But as your power increases, you also begin to feel a detachment from the living. From this point on, you have disadvantage in all charisma ability checks except for the ones performed for intimidation. You also gain vulnerability to radiant damage, since your body now is overtaken by profane energies. From now on, your body has been subjected to such an amount of prophane energies that rejects any good-willed intervention. You won't benefit from any source of healing other than your own spells or resting. In addition, if you receive a healing effect from a spell with divine origin, you take damage equal to the amount that should've been healed.

You new visage will cause people to react to you in different ways. Some may fear you instinctively, refrain from talking to you, shun you or even attack you on sight.

Lord of undeath

From level 15 onwards, by using the souls you ripped from the living, you create a mockery of life.

You may cast the spell Create Undead as a level 6 spell, increasing by 1 level per extra soul invested. You will not use spell slots, but you'll lose the souls from your count. In order to maintain control over this undead creatures for more than 24h you'll have to invest one extra soul per day in order to power your dark powers.

Hunger Frenzy

Beginning at level 15, when you drop below 25% of your maximun hitpoints you make a wisdom saving throw. You may choose to fail this saving throw.

On a failed saving throw, you enter a rage-like state of blind hunger. You drop your weapon and your attack action is replaced with a melee drain spell which you can perform as a melee attack. The damage of this spell equals twice the damage of your Life Leech feature, and your healing is always 3 times your constitution modifier.

Upon entering this state, you must make another wisdom saving throw. If you succeed, you gain normal control of your PC and can choose whether to move, attack or do any other action. If you fail, you must move to the closest alive creature and attack it with your drain spell. You must perform this saving throw at the beginning of each of your turns while you are under the effects of this feature.

If you are under the effect of any spell (such as barbarian rage) that prevents you from concentrating, you automatically fail the wisdom saving throw. This frenzy lasts for 3 rounds, and ends inmediately if the battle ends. If you wish to extend the duration for longer than 3 rounds, the hunger begins to control you mind and from the 4th round on, all of your wisdom throws to keep in control have disadvantage.

Undying body

You have attained total mastery over the forces of death, and you may use your last strenghts to cling to life at all costs. Beginning at level 18, if you're reduced to 0 hitpoints and don't die outright, you automatically move to the closest alive creature and perform a Hunger Frenzy drain spell (twice the damage of your Life Leech feature, and 4 times your constitution modifier of healing). After feeding off the life of this creature, your frenzy stops and you regain control of your character for the remainder of your turn.

If you find yourself unable to drain any living creature, you fall unconscious instead. You can't sustain your body without life force.

You can use this feature a number of times equal to your wisdom modifier per long rest.