Soul points

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 Slot.
Soul Points = [(5 + Wisdom Modifier) x character level] + spell slots
The following chart makes it easier to determine how many slots a character has per level.
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SOUL POINT LOSS
Characters will lose their precious Soul Points every time they deal with supernatural events, dread manifestations of Ènferun and when attacked by creatures of the dark.
Characters reaching 0 Soul Points become Possessed: a new condition described below.
Here are the most frequent causes for Soul Point Loss:
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 Spellcasting: Whenever a character casts a 1st level spell or greater, they lose 1 Soul Point, unless it is a Holy Spell.
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 A Holy Spell is defined as any spell that deals radiant damage, or by the presence of the words “holy”, “divine” or “faithful” in the spell’s name; in addition, all spells from the School of the Moon are to be considered Holy.
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 Vile Foe: Whenever a character faces Aberrations, Fiends or Undead, they must make a successful Wisdom Saving Throw (DC 10 + half the creature’s Challenge Rating, rounded down to a minimum of 10) or losing as much Soul Points as the creature’s Challenge Rating (to a minimum of 1; further encounters with the same kind of creature during the same day involve no further Saving Throws.
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Might of the Soul: As a bonus action, a character can sacrifice their Soul Points, from a minimum of 1 to a maximum equal to their level plus their Charisma modifier, to obtain as many bonus Temporary Hit Points, that last until spent; however, a character cannot willingly sacrifice all their remaining Soul Points using this ability.
Overwhelming Temptation (optional): Whenever a character indulges in sin, allows themselves be tempted by darkness and befittingly portrays a behavioural flaw, they can choose to roll 1d4, 1d6, 1d8, or 1d10 to determine how many Soul Points they can trade for a bonus to any Ability check, Saving Throw or Attack Roll performed in the following turn; the name Overwhelming Temptation suggests that it is up to the DM to provide an opening, such as an ethical bind or too tempting an opportunity, for the character to show their deplorable side and act out accordingly. This mechanic is intended to be optional: using it in game sessions or not should be discussed by the DM and the players prior to the game.
A Taste of Darkness (optional): When brought to 0 hit points, a character can use their last moments of clarity to linger in the surrounding darkness and delve into their own soul to find the same darkness. By doing so, they can opt for losing Soul Points equal to their character level to stay at 1 hit point instead of 0; once used, a character can resort to Taste of Darkness only after a Short or Long Rest.

Evil Creature Assault: Most creatures from Ènferun add spiritual damage to the physical one, causing Soul Point loss. Any creature listed in the Bestiary has the amount of spiritual damage it deals reported in its statistics; however, if a NM wanted to employ creatures of its own creation or taken from another rulebook, the following guidelines might help them determine the amount of spiritual damage dealt per round according to the monsters’ Challenge Rating.
Note: Not all monsters have to deal spiritual damage; if every fight involves this kind of game mechanic it might become boring, predictable, or needlessly deadly.
Challenge Rating of 1 or lower: 1 or 2 Soul Points lost per round
Challenge Rating from 2 to 4: 3 (1d4) Soul Points lost per round
Challenge Rating from 5 to 7: 4 (1d6) Soul Points lost per round
Challenge Rating from 8 to 10: 5 (2d4) Soul Points lost per round
Challenge Rating from 11 to 14: 7 (2d6) Soul Points lost per round
Challenge Rating 15 to 17: 10 (3d6) Soul Points lost per round
Challenge Rating from 18 to 20: 12 (5d4) Soul Points lost per round
Challenge Rating over 20: 15 or more (6d4 or 4d6) Soul Points lost per round

Narrative Soul Point Loss: From time to time, the NM might need to inflict a Soul Point loss to increase tension and put the characters under pressure. Any NM should find a kind of balance fit for his party, keeping in mind that Soul Point loss is the primary cause of death in Nightfell, because a Possessed character with 0 hit points dies instantly.
Narrative Soul Point losses per day should approximately be equal to: character’s level + 2
Example: Over the day (between one Long Rest and the
next one), a NM dealing with 2nd level characters should inflict a most a Narrative Soul Point loss up to 4 points; if the characters were 10th level, the narrative Soul Point loss could sum up to 12 points.

REGAINING SOUL POINTS
A character’s spiritual fortitude can be regained by meditating or simply committing to small things and earthly matters, to rediscover life’s bright side and momentarily forget about the looming darkness.
After they complete a short rest, a character regains 1 + Wisdom modifier (to a minimum of 1) Soul Points for every spent Hit Dice.
NB: A character with full Hit Points can still spend their Hit Die to regain Soul Points in this way.
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: an intelligent character will try and understand, learn from their mistakes, and rationalize their fears; a wise character will follow a philosophical or a religious path, reassuring themselves with thoughts of transcendence and higher purposes; a charismatic character will try and find their inner strength, their resolve to go on and overcome any adversity.
Other ways to spiritually recover, such as the Lunar Blessing, are explained in the related paragraphs below.

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 Saving Throw vs. Death, 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.
A Possessed character is 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 Interdiction from Evil (a ritual that can be celebrated by any character, yet often needs more officiants, and could pose risks for both those trying to remove the Possession and the Possessed one), or alternatively a Lunar Blessing, specialty of the Lunar Cultists.
Below, a recap:
• Possessed: the character cannot recover Soul Points; if reduced to 0 Hit Points, they die (without any Saving Throw vs. Death); any time they are supposed to lose further Soul Points, they gain an Exhaustion level, instead.
PORTRAYING A POSSESSED ONE
A Possessed character needs the player to 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.
These guidelines remind of the Overwhelming Temptation (see above) yet taken to extremes and prolonged over time.
In a similar way, a merciful NM could reward a player thoroughly portraying their character’s possession with a 1d6 roll result, usable only once per Possession state, to add as a bonus to any A bility Check, Saving Throw or Attack Roll.

INTERDICTION FROM EVIL
This ritual requires an hour to complete and a variable number of participants: generally, the more characters who are involved the lesser the risks.

Characters’ perspective
The characters bind the Possessed one with a rope, they put them on their knees and then they draw a circle of salt around them in the shape of an apotropaic circle.
The characters place themselves on the circle, then they take turns reciting the first couplet of the Prayer for Interdiction. If the prayer is successful, the ritual continues;
if not, the Possessed one and the officiant are trapped in an dark vortex that might drain their very soul, unless another
officiant resume the ritual from when it failed.
When all four couplets are recited successfully, the Possessed one is relieved from their condition, and all of the characters can finally rest.
If one of the officiants allows themselves to be drawn into the inner darkness of the Possessed one or loses all their Soul Points the ritual fails, and the officiant becomes Possessed themselves (this particular kind of  Possession arises suddenly, violently, and dramatically
and the newly Possessed could try to harm or kill their
companions, especially the one undergoing the ritual, or
unleash their most vile urges and flaws).
NB: No bonus for a proper interpretation should be
awarded by the NM in the case of total failure. If
deemed necessary, the NM could even suggest a course
of action to the player or rather temporarily taking
control of the character.
The ritual requires at least three officiants, three as the
Lunar Phases (not counting the New Moon, dark and
shadowed, symbolized by the Possessed one placed
at the centre of the glyph. If the officiants try to back
out and the ritual is interrupted before completion,
the Possessed one dies instantly, while the officiants
feel their souls torn away and dragged into Ènferun,
incurring themselves in the risk of Possession.

Players' perspective
The ritual requires at least three participants. As
long as three characters abide and stay in the circle,
the ritual may carry on. When their turn comes, any
officiant may decide to leave the ritual. If the ritual is
interrupted because fewer than three officiants remain,
the Possessed one will die instantly, whereas all the
officiants lose 1d6 Soul Points and an additional 1d6 for
every couplet successfully recited so far.
The players agree upon an order to recite, which must
be followed throughout the ritual. In turn, players roll
an Intelligence (Religion) check, DC 10 + 1 for every
previous attempt (be it success or failure).
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(optional rule: if you want to add further difficulty and
suspense to the game table, you can employ a rule by which
the dice must be rolled on the Hex Glyph on the Possessed
one’s sheet; if the dice rolls out of it, the attempt fails
automatically)
Success: The player/character may recite the following
couplet (two verses). Once the fourth couplet is closed,
the ritual succeeds.
Failure: The Possessed character gains an Exhaustion
Level, the officiant loses 1d4 Soul Points for each
preceding couplet minus their Charisma modifier. The
following character restarts from the failed couplet, not
yet correctly recited (DC is increased by 1).
Once every couplet has been recited correctly and in an
orderly fashion, meaning the party got four successful
rolls, the ritual ends, and the Possessed character is
relieved from their wretched state, regains 1 Soul Point
and will be able to recover the Exhaustion Levels and
Soul Point loss in the usual ways.
If one of the officiants reaches 0 Soul Points by failing
in reciting their couplet, the ritual fails, and said
officiant is in turn Possessed (such Possession often
manifests dramatically, violently, and suddenly: the new
Possessed one may try to kill the other Possessed one, or
turn against their companions, or give in destructively
to their fear and anger or other negative impulse).
NB: In such a disastrous circumstance, even a successful
interpretation should not be awarded by the NM, whom
could even suggest a different course of action to the player or
temporarily take control of the character.
Here, the ritualistic formula:
“Come unto us, o moon light,
may thy splendour guide our rite.
Spawn of void, accursed Fiend,
to leave this body, our prayer bids.
Light be still, stand the will,
may life win whatever ill.
Thus, we exile thee through the door
of a dark mirror forevermore.”