Path of the Unholy

Many fear the Unholy, as their impulses of wrath are punctuated with madness, which makes them as unpredictable as they are unreliable. The heat of battle takes over in these warriors, blinding them to danger with the sacrilegious darkness that bloats their hearts and moves their arm. 

These characters draw strength and fervor from their own soul's distress. The descent into the oblivion of madness awakens violent and obscure powers along with the insatiable desire to use them: a desire to which they often decide to succumb. Those who choose to follow this path of brutality and insanity will take strength from the approach of the end, and will weaponize the corruption of their spirit. 

The following information can be found in the Nightfell 5e supplement Pgs. 148-151

SOUL POINTS
What Are They?
Soul Points represent a character's psychological and spiritual fortitude in the face of soul-consuming decay. Each character has a Soul Point supply of 5 plus their Wisdom Modifier per level. In addition, spellcasters gain 1 more point for every Spell Slot they have. Exceptionally, Warlocks gain 3 points for Spell Slots.

Soul Points = [(5 + Wisdom Modifier) x character level] + spell slots

REGAINING SOUL POINTS
After they complete a short rest, a character regains 1 + Wisdom modifier (to a minimum of 1) Soul Points for every spent Hit Die.
After they complete a long rest, a character regains an amount of Soul Points equal to their highest ability among Intelligence, Wisdom, and Charisma. 
Such versatility represents the way different characters deal with trauma in their sleep.

POSSESSION
When a character reaches 0 Soul Points they become Possessed, haunted by evil spirits who try to lure them to their death. If a character reaches 0 Hit Points while Possessed, they will not be able to resort to a Death Saving Throw, thus meeting their doom with swift simplicity.
If confronted with a situation that entails Soul Point Loss, the character will instead gain an Exhaustion Level. Possessed characters are unable to regain Soul Points in any way.

The only way to get rid of Possession is to undergo an adequate Cleansing Ritual such as an Interdiction from Evil, a Lunar Blessing, or magic that specifically targets possessed creatures (such as Dispel Evil and Good).

PORTRAYING A POSSESSED ONE
A Possessed player should display their character's worst vices, flaws, and whatever resides in the darkest edges of their heart, with grim morbidity. Their identity twisted yet faithful to themselves. Bravery turns into recklessness, fear into aggression, mistrust into xenophobia and contempt, justice into thirst for vengeance. A knight bent on vanquishing evil becomes a merciless slaughterer dwelling on absolutes.

Unholy Thirst

At 3rd level, you learn to yield to the temptation of darkness, forgoing fragments of your soul in exchange for a promise of power that can only really be unleashed when pervaded by rage.

At the beginning of each turn while in Rage, you may choose to lose a Soul Point: in doing so, the next Strength-based attack with a melee weapon will deal 1d12 additional damage. If the attack is not enough to reduce the target to 0 Hit Points, you take 1d6 necrotic damage that cannot be resisted in any way.

Blood of the Soul

At 3rd level, you learn to draw power from the darkness instead of being swallowed by it. When you suffer an involuntary Soul Point loss, you can use your Reaction to gain 1d12 Temporary Hit Points and have Advantage on Saving Throws until the end of your next turn.

On the Verge of Oblivion

At 6th level, you begin to feel more comfortable on the verge of oblivion; one step away from seeing the darkness of your soul reunited with those of the Dark Mirror. 

While your Soul Point total is below your character level, you ignore the first 5 levels of Exhaustion (the sixth is fatal anyway).

Dark Devourer

At 10th level, you get nourishment from the essence of the dark creatures that die in your vicinity. If you were in Rage within 30 feet of an Undead, Fiend, or Aberration when it drops to 0 Hit Points, you regain an amount of the Soul Points equal to the creature's challenge rating.

Unholy Wrath

At 14th level, your wrath permanently devours what remains of your soul: entering Rage involves a loss of Soul Points equal to your Constitution modifier, and you add your Constitution modifier to the damage inflicted with melee weapons as long as you are in Rage.