I was struck with inspiration as I lay in bed last night. I'm gonna write it as a rule for dnd 5e but the concept could be adapted to similar games easily.
The basic idea is to sometimes replace damage with Corruption Dice (spooky), which are subtracted from d20 rolls and sometimes cause surprise damage.
(plain text is rules jargon, purple text is me saying stuff)
This mechanic is meant to emulate various forms of magical curses as an alternative to damage. In my head it's something that can be inflicted on players if you want to create a sense of impending doom without just damaging them.
It adds a numeric way to simulate indirect harm that spooky things might cause. Just damage can get boring so this is something to heighten vibes and force players out of their comfort zone. The intended effect is to make players feel like something bad is coming, or like they're being forced by some supernatural energy to accept harmful consequences.
Added bonus is that it can mitigate insane modifiers that high level players end up with.
Corruption is inflicted on characters (or creatures generally) as a number of dice. These dice are not rolled but are added to a pool. For example, if a creature takes 3d6 corruption, add 3d6 to their total corruption and don't roll them (this could be a written tally or a kept as a pile of dice in front of the player). A creature with a non-zero amount of corruption is referred to as corrupted.
This pool of dice remains until spent, or until removed by the lesser restoration spell or any effect that removes curses. Whenever a corrupted creature makes an attack roll, ability check, or saving throw, they can spend some of it by rolling a number of corruption dice and subtracting the total from the initial roll. When they spend corruption in this way, they cannot spend more than 3 dice on any given d20 roll.
Alternatively, a corrupted creature can spend up to 3 corruption dice during a short rest, roll them, and take damage equal to the total.
When a corrupted creature makes an attack roll, ability check, or saving throw and rolls a natural 1, they must immediately spend all of their corruption dice at once. When the dice are spent in this way, they deal necrotic damage to the corrupted creature equal to the number rolled.
Corruption can be quickly inserted as a replacement for damage. It can appropriately be inserted into the spells or abilities of fey, fiends, or undead. One use I would recommend would be to use it instead of damage for magical traps or include the mechanic when designing cursed magic items. For a direct conversion, consult the following table:
If this makes it more complicated than necessary, or you don't want to mix types of dice, just do 2d6 for every 5 damage.
This is a good place on a stat block to sub in or add corruption. In addition to damage conversion, you may replace any condition caused by a special action (such as frightened or paralyzed) with a number of d12s equal to the challenge rating of the creature. If you wish to make a creature more powerful, choose one or two of these actions and add d6 corruption/CR to the existing effects (allowing a wisdom or charisma save to avoid it if you want).
Unhallowed land, the shadowfell, the territory of a cursed or undead creature, or the domain of an illegitimate ruler may all be places where corruption permeates the air and soil. Use it when it feels right or for a specific antagonist or faction if you want them to have a unique flair. When a creature finishes a long rest in a corrupted place, they must make a DC 15 Charisma saving throw. On a success, the creature feels a vague sense of unease but suffers no other effects. On a failure, the creature gains 2d6 corruption.
If the source of the corruption is a sentient creature, they can choose up to 13 creatures that they know to be within the affected area. When any of those creatures fails their saving throw, they instead gain 1d12 corruption for each night they have spent in the affected area.
Following are a few example traits that use corruption. They can be added to the statblocks of appropriate creatures. Alternatively, they can be used as identifying features of all creatures in a specific faction or from a specific place.
Aura of Evil. Any creature that starts its turn within 10 ft. of [this creature] must succeed on a DC [8 + proficiency + Cha or Con] Charisma saving throw or gain 1d12 corruption.
Corrupted Strikes. [This creature]'s weapon attacks are magical. When [this creature] hits with any weapon, the target gains 1d8 corruption. A creature immune to necrotic damage or curses is immune to this effect.
Foul Magic. Any creature that fails a saving throw against a spell cast by [this creature] also gains 1d8 corruption. A creature immune to necrotic damage or curses is immune to this effect.
Tainted Blood. A creature that touches [this creature] or hits it within a melee attack while within 5 ft. of it gains 3d6 corruption.
Below is a single example magic item, to show the way this mechanic could be used to represent curses.
I think it's neat because the ability to use corruption to your advantage encourages a player to seek out dark and dangerous situations. It makes corruption a little less scary, but it uses the meta to push the character to find opponents to inflict it on, lest they suffer the consequences of their actions.
In darker campaign settings, corruption can be used in roleplay to underline thematic elements. The DM may inflict 1-3d8 corruption on any character who gives in to dark desires, betrays someone's trust, does something impious or blasphemous, or witnesses something grim and horrifying.
The DM and players should decide ahead of time exactly which actions or events will lead to corruption, and how severe they have to be to incur a given number (or size) of dice. Consider which type of roleplay you all want to encourage, and which themes should be emphasized.
Spells and abilities that deal damage can easily be converted, either on a character by character or spell by spell basis. Spells that this change would be most appropriate for include arms of hadar, blight, chill touch, cloudkill, dissonant whispers, summon celestial, summon undead, toll the dead, etc.
You could also modify (or make up!) class features so that they inflict corruption instead of damage, or in addition to other effects. A vengeful Cleric's Channel Divinity might inflict corruption upon sinners, even a Rogue's sneak attack might inflict supernatural woe in place of additional damage.
If you, as the DM, allow players to cause enemies to gain corruption, remember that you should spend it more liberally than a player might. You can and should roleplay as that monster or enemy, but consider that a creature that will likely not survive the fight may make the players feel cheated if it never chooses to suffer the penalty.
The damage type that corruption deals could easily be changed to highlight different themes. Change it to psychic damage for a more horror or lovecraftian setting, change it to poison damage it you are using it to represent toxic substances or diseases. Perhaps you want to use it to represent radiation poisoning, in which case it could be radiant damage (if not poison or necrotic).
Another idea I like is to make the corruption represent horrible luck. Because the damage is triggered by natural 1s, the DM can describe it as a consequence of some kind of accident and declare an appropriate damage type. For example: a character that rolls a natural 1 on an attack may fall on their sword and take piercing damage equal to their remaining corruption, or a natural 1 on the Constitution save triggered by a green dragon's breath may represent a character gasping and breathing in additional poison damage like an idiot.
Another change, for a more sword and sorcery setting, could include allowing spellcasters to accumulate corruption in exchange for more potent magic. Whenever an arcane caster casts a spell, they may gain 1d12 corruption and increase the level of the spell by one without expending a higher level slot. This could also be a mandatory feature, to make all magic a form of dark bargaining.
This has not been play tested so you may also find it necessary to balance it by including maximum corruption (perhaps dealing its damage if it reaches that threshold), limiting it to d6s or even d4s, or making it less deadly. If the potential damage seems too high, random, and/or disruptive to combat balance, it could be spread out. To do so, change it so that each time a corrupted creature takes damage, they spend one die and add it to the damage.
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