Oath of Sin

To many you'd be observed as a criminal. One either with a sliver of a chance for redemption or one who is simply the abyss incarnate. No matter what end of the spectrum you fall on, there are many who will be forever wary of you. It is because of this reputation you've cultivated that you found yourself in servitude to this creature. A King of Hell. A title that would draw the ire of any who even held a shred of knowledge for what that meant. Yet to you, you saw opportunity. Someone who held influence, authority, and power and wished to cultivate a fraction of it for your own ends. What is it you wish to fulfill using these abilities? Well . . . . that is up to you to decide.

As you take this solemn oath, your tenets you swear change based on your alignment:

Neutral
The personal follies of others is of no consequence of yours. You act in the interest of your own ambition with all other requests come secondary to yours. If a deal can be made, don't waste the opportunity to turn it to your benefit.

Good
Forever seeking penance, you act in a manner that would best serve towards your salvation. Absolution is your aim and only through the devil can you hope to achieve it. A paradox to many but you know it was the only route available to you.

Evil
Destruction, death, and chaos. Those are your values and principles. You're wilder than the devil himself but still you are subservient to them. Through respect or mutual ideologies, your reasons are your own. 

Being a Tiefling grants you additional abilities in this subclass, as a Hirudinea you are granted even more. If you are Hirudinea and take this subclass, multiclassing into, or out, permanently halves your max HP and restricts you to only the Tiefling benefits.

Level 3: Oath of Sin Spells

The magic of your oath ensures you always have certain spells ready; when you reach a Paladin level specified in the Oath of Sin Spells table, you thereafter always have the listed spells prepared.

Oath of Sin Spells

Paladin Level	Spells
3	Cause Fear, Devil's Due
5	Death Armor, Mirror Image
9	Blink, Crimson Harvest
13	Banishment, Murder of Crows
17	Scrying, Steel Wind Strike

Level 3: Mark of Sin

You can expend a charge of your Channel Divinity as a bonus action to swaddle yourself in an aura of sin. This aura appears around you as the lost souls of the damned. The aura stays active for 1 minute or until you are rendered incapacitated. While active, you are able to mark creatures you hit with a weapons attack. Creatures marked this way gain a Sin Tally. These marks can only be placed from your first successful attack. You can stack a number of Sin Tallys on a creature equal to your Intelligence modifier. As a Magic Action you can enflame each tally and deal the amount of stacked Sin Tallys in d8s as necrotic damage.

Tiefling or Hirudinea
When you, or another creature, casts a spell that deals damage immediately upon cast; you can use a reaction to enflame any number of Sin Tallys should the creature targeted by the spell have any.

Hirudinea
When you enflame your Sin Tallys you gain an amount of Temporary Hit Points equal to the number of necrotic damage dealt.

Level 7: Aura of Malcontent

A creature benefiting from your Aura of Protection is considered Enraged for as long as they stay within the radius. Creatures with this condition have disadvantage on saving throws against ones that use either Wisdom or Intelligence. In exchange, creatures with this condition deal an extra 1d4 necrotic damage as well as gain resistance to either fire, necrotic, lightning, or force damage. You choose this damage when you give out your aura and it can't be changed till after you've taken a long rest.

Tiefling or Hirudinea
Creatures in your aura also have immunity to the Charmed condition.

Hirudinea
Creatures Enraged from your Aura of Malcontent can also gain resistance to either slashing, or piercing damage in addition to the other resistances it might get.

Level 15: Sins and Deeds

Creatures that bear your Sin Tally cannot further taint those within your presence. Should a creature that is marked attack an ally that is Enraged by your aura with a natural weapon, they now deduct half your charisma modifier + half your intelligence modifier + half your proficiency bonus, all rounded down, from their attack roll.

Tiefling or Hirudinea
This debuff can now affect a creature's saving throw but, choosing to have the deduction affect the saving throw mean's it can't affect the creature's attack roll. The saving roll deduction also doesn't trigger unless an Enraged creature is the one who forces the save from the targeted creature.

Hirudinea
You can now expend a Hit Die to further enhance the debuff. The expenditure requires no action and you can choose to expend as many as you want. However, a Hit Die can only affect one attack roll, or saving throw and, when expended, your max Hit Points is reduced by three times the roll of your hit die regardless of any effect that might prevent such a thing from happening.

Level 20: Necrotic Spawn

Its at this stage you achieve a more symbiotic relationship with your liege lord. By expending a Channel Divinity as an action, you become momentarily bathed in necrotic light that causes your physical form to shift and change. Long twisted horns grow from your forehead, your skin sheds and becomes more reptilian in appearance, your teeth turn razor sharp and your nails grow and become like pointed knives. This form lasts for 10 minutes, or unless you become incapacitated. While this form is active you gain the following effects:

- Any creature within the radius of your aura must make a Charisma save, DC equaling your spellsave DC, where on a fail they take necrotic damage equal to your Charisma modifier. This save happens at the beginning of a creature's turn.
- Any necrotic damage you deal ignores necrotic resistance.
- Sin Tallys you place now have their damage die size increase from a d8 to a d10
- Your unarmed strikes can deal either slashing, or necrotic damage and deal 1d6 in addition to the damage it normally deals.

Should this form ever end prematurely, you take 3 levels of Exhaustion regardless of any immunities you might have, as well as damage from any Sin Tallys still in effect regardless of any immunities to necrotic damage.

Tiefling or Hirudinea
When you activate this form, you also gain a flying speed equal to your movement speed as well as being able to add half your Intelligence modifier to your attack rolls.

Hirudinea
While this form is active, and you have at least one creature Enraged your Radiant vulnerability is made null.