Dread and Insanity

The rules described here are a variant of the rules provided in Sandy Petersen's Cthulu Mythos supplement. The rules presented here both add and remove rules from those options. For a more detailed explanation, or to see the ruleset in it's entirety, purchase the supplement or ask your DM if you can have a copy of the ruleset in it's entirety.

A variant ruleset you can apply to your campaign involves the implementation of the Dread and Insanity system from Sandy Petersen's Cthulu Mythos supplement. This system revolves around making Saving Throws against effects that warp the mind and increase in strength until a character can no longer bear the effects and is driven mad.
The ruleset is quite complex, but the following is a very barebones explanation of how dread and insanity function under these rules.

Dread

Dread is fundamentally different from fear/fright. Dread encompasses profoundly disturbing experiences—experiences that shake the foundations of the mind and spirit. Dread is measured in seven increasingly debilitating levels that are displayed in the table below.

Level	Effect
1	Disturbed. Must succeed on a Wisdom saving throw against the dread’s save DC to move closer to the dread’s source
2	Spooked. Cannot approach the source of the dread (automatically fails the saving throw listed at level 1 disadvantage on ability checks while the dread’s source is within line of sight
3	Afraid. Disadvantage on attack rolls while the dread’s source is within line of sight
4	Staggered. Insanity risk (see below drop all held items upon reaching this and any higher level of dread; can’t take bonus actions or reactions; at the start of own turn, must make a Wisdom saving throw against the dread’s save DC; success reduces dread level by one
5	Panicked. Must take the Dash action and move away from the dread’s source by the safest available route on each turn unless there is nowhere to move
6	Paralyzed. Paralyzed (see the condition) with fear, either in immediate prior position or cowering
7	Faint. Fall unconscious from overwhelming shock; must succeed on a Constitution saving throw against the dread’s save DC or die; a successful DC 15 Wisdom (Medicine) check awakens the victim and reduces dread level by one

An effect can give a creature one or more levels of dread at a time, as specified by the effect or DMs discretion. Each dread condition has a save DC used for Saving Throws against its effects, generally matching the DC of the inflicting effect.
Dread effects are additive: If a creature has one or more levels of dread when it gains dread, its current level increases by the amount specified in the effect's description. The longer of the new dread’s duration and the preexisting dread’s remaining duration applies to the new dread total. The dread condition’s save DC becomes the higher of the preexisting dread’s save DC and the new dread source’s DC. A creature suffers from the effect of its current level of dread as well as all lower levels.

Removing Dread

An effect that removes the frightened condition removes three levels of Dread unless otherwise specified and other spells and effects that calm emotions or soothe them might reduce dread at the DMs discretion.
When a level of dread is removed, a creature no longer suffers from the effects of the removed levels and all dread effects end if a creature's dread level is reduced below 1.
Dread can also fade on its own over time. Sources of dread may have a specified duration, at the end of which the level of dread in an affected creature reduces by 1 level. For effects that don't list a duration, a creature's dread is reduced by 1 level when it finishes a long rest.
Dread and the Frightened Condition. Nonmagical effects that cause the frightened condition (such as a dragon’s Frightful Presence) don’t necessarily cause dread. A creature can be frightened and experiencing dread at the same time without those conditions interacting with each other

Supernatural Influence

Spells and other magical effects that would cause the frightened condition instead cause 3 levels of dread. Spells and other magical effects that cause insanity cause dread in addition to their usual effects; the level of dread is equal to half the spell's level, or one-quarter of a creature's challenge rating for magical fear produced by a trait or action.
     Dread from magical fear or from the same creature/kind of creature isn't cumulative. Only the highest level of dread and longest duration from among those dread conditions is used. If the spell or magical effect doesn't have a save DC, use DC 15 or another appropriately determined DC.
     A dread effect caused by a supernatural influence lasts for as long as the associated insanity or frightened condition would normally last before decreasing.

Insanity

Insanity is a long-term condition that has specific effects on a character and their abilities. Insanity is less common and immediate compared to dread, but it is potentially devastating over the long term.

Insanity Strength

Insanities have a save DC that represent its strength. This save DC is equal to the save DC of the source of the insanity. If the source doesn’t have a save DC, use DC 15 or whatever DC would be most appropriate if one were allowed.

Multiple Insanities

Whenever a character gains a new form of insanity, the save DCs of any existing insanities each increase by 2. If the new insanity gained is one the victim is already suffering from, use the higher DC between the two + 5 instead of the character having two instances of the same insanity.

Recovering from Insanity

Once every 10 days, a character with an insanity can attempt a Sanity saving throw against the insanity’s DC. On a success, the insanity’s DC is reduced by 1d4; note that some forms of insanity have effects that trigger if the character fails this save. As long as the insanity has a DC of at least 1, the character continues to suffer the full effects of the insanity, althoughit becomes easier to resist its effects and recover as the DC goes down. Once the insanity reaches a DC of 0, the character is cured and that particular insanity no longer affects them.
Insanity can be treated by ability checks using the Insight or Medicine skill. First, the insanity must be identified. A character can identify an insanity with a successful Wisdom (Insight or Medicine) check against the insanity’s save DC. Then, with 1 hour of effort per day for 10 days, the healer can treat the insanity with a Wisdom (Insight or Medicine) check against the same DC at the end of the week. If the check succeeds, the patient receives a +2 bonus on their saving throw to reduce the insanity’s save DC for that 10-day period.
The Greater Restoration spell MAY cure one insanity afflicting a creature. If a target creature has multiple insanities, the spell only affects the insanity with the lowest DC (chosen by the caster among identified insanities in the case of a tie, or randomly if one isn’t specified).

Sources of Insanity

There are four main ways in which a character can be driven insane by events that occur during a game: overwhelming dread (having dread of level 4 or higher), catastrophic mental trauma (mental ability score reduction), confusion effects, or effects that directly afflict the character with insanity.

Overwhelming Dread

Whenever a character reaches dread level 4 or higher, that character must make a special Sanity saving throw against the dread’s save DC. On a failure, the character gains an insanity related to the source of dread. Some sources of dread add multiple levels of dread at once. If a character gains multiple levels of dread resulting in a total dread level of 5 or higher, they make that saving throw against insanity with disadvantage.

Catastrophic Mental Trauma

When a characters Intelligence, Wisdom, or Charisma scores suffer a reduction, they must succeed on a Sanity saving throw or gain an insanity related to the cause of the ability score reduction. The DC is equal to 10 + the value of the reduction. If a character’s Intelligence, Wisdom, or Charisma score is reduced to 1 or lower, that character automatically develops such an insanity.

Confusion Effects

When a character is affected by a confusion spell or another spell or magical effect that causes the character’s actions to be determined randomly, roll a d100. On a result less than or equal to the save DC of the spell or effect, the character gains an insanity that persists after the spell or effect ends.

Other Insanity Effects

Rather than becoming unable to act or communicate, a character who fails to save against the detrimental effects of a contact other plane spell or a symbol (insanity option) spell immediately gains a specific form of insanity chosen by the caster.