Many common folk have fell of hard times, and wish for the pain of suffering to go away. Some wish to delve their traumas down into the depths of the shadows, never to return again. It is here where Shar, the Lady of Loss, finds her devotees. No priest of the Dark Lady goes to her guide entirely willingly - it is of desperation of the warm comfort of the night. The deity may change between worlds, or maybe lack thereof and be created from the extreme devotion to committing despair, but the oath remains the same throughout time - it is a reflection of dread power, and those who choose to throw themselves to the Lady of Loss are solely to inflict despair in return for their own pain in the name of their devoted. Such inspirations to join these ranks differ between paladins, some not even evil, but rather wanting to spread grim truth, but nonetheless, despite whatever past they made have had, they are now hollow, depressive bringers of the night. 

Many paladins who follow this order do not do their deeds under the sun or on nights of a full moon. On new moons, however, it is often ritualistic of them to commit an act of evil with proof to the congregation, however small that may be. They often cloak themselves in dark colors or even go under the guise of a paladin of another order in order to avoid persecution. 

Tenets of the Shadowheart

The tenets of the Oath of the Shadowheart depends on the worship and the connection to their cause, but the following four principles lay the groundwork;

Foster Strife. Wherever you can, encourage the feeling of pettiness and regret to form the all-powerful emotion of despair, with whomever you meet.

Conspiracy. You must always expect the worse to come. Never let anyone come more than an arm's length - the only warmth you will find is amongst the Dark Lady.

Hopelessness. Never turn to hope, or follow promises of success. They are but fallacies to deny the real, bloody, cold truth. Revealing this is more merciful than lies. 

Seal the Light. Quench the light and love of the world whenever you find it, and hide when you cannot prevail.

Oath Spells

You gain oath spells at the paladin levels listed.

3rd - dissonant whispers, hex

5th - darkness, gift of gab

9th -  vampiric touch, bestow curse

11th - shadow of moil, phantasmal killer

15th - dominate person, modify memory

Smites of Despair

Fueled by bitterness and contempt rather than holy right, your smites are made to bring down only the most primal of defenses - pure, crushing, despair. When you use your Divine Smite feature, the damage type is changed from radiant to force. Additionally, instead of undead or fiends, the damage increases by 1d8 if the target is a fey or a celestial, to a maximum of 6d8.

This also applies to your Improved Smites feature, changing it to force damage.

Channel Divinity - Cloak of Darkness

You can use your Channel Divinity to vanish into the unseen. As a bonus action, you become invisible for 1 hour. You become visible if you attack or cast a spell.

Channel Divinity - Invoke Misery

You can incur a sense of doom within your foes, draining their hope. As an action, you may force a number of creatures of your choice equal to your Charisma modifier (minimum 1) within 30 feet of you to make a Wisdom saving throw. On a failed save, they are affected as if they were under the Slow spell for 1 minute. 

Aura of Shadows

Starting at 7th level, you start to blend within the Shadow Weave crafted by the Dark Lady herself. When you, or any friendly creatures of your choice within 10 feet of you, are in an area of dim light or darkness, you can use your action to become invisible until you move, or take an action or a reaction.

At 18th level, the range of this aura increases to 30 feet.

Strands of the Shadow Weave

Starting at 15th level, you can step between the strands of the weave to negate attack by those who dare to defy you. Immediately after you are hit by an attack, you can use your reaction to turn invisible and teleport up to 60 feet to a spot you can see. You remain invisible until the end of your next turn or until you attack, deal damage, or force a creature to make a saving throw. You may perform this reaction up to 1 + your Charisma Modifier (Minimum 1) times before you take long rest when you can use it again.

In addition, ordinary and magical darkness no longer impedes your vision, and you take no penalty to Perception rolls due to dim light.

Form of the New Moon

At 20th level, as an action, you can become an embodiment of despair, channeling the Shadowfell from your pure faith. For 1 minute, you gain the following features;

All attacks have disadvantage against you while in dim light or darkness, as your form is uncertain within the shadows.

You reduce any bright light in a 30 foot radius to dim light, and dim light to magical darkness.

You gain a bonus to your weapon attack rolls equal to your Charisma modifier.

While the transformation lasts, the paladin can use a bonus action on his or her turn to cause the shadows in the aura to attack one creature. The paladin makes a melee spell attack against the target. If the attack hits, the target takes psychic damage equal to 3d10 + the paladin's Charisma modifier.

Once you use this feature, you can't use it again until you finish a long rest.