Shadow Sickness

A supernatural disease that strikes those who are wounded within areas of supernatural darkness or places corrupted by Shadow.
Based on this for inspiration. (Link leaves World Anvil)

Causes

When a creature takes 25 or more damage when in a Shadow Corrupted place or within magical darkness (such as the darkness spell) the creature has a 10% chance to contract the disease. The chance increases by 5% per subsequent occurrence within 10 minutes of the first. At midnight while the victim is infected, it must make a DC 15 CON save. On success, nothing happens. On a failure, the victim takes 1d4+1 necrotic damage that ignores resistance and immunities.

Symptoms

• Pale Skin
• Inescapable Chills
• Lowered Body Temperature
• Lowered Vitality and Strength
• Dark Black Wounds

Treatment

There is no known cure, but their are ways to mitigate the effects until the magic wears off.

Prognosis

As the days go on, the victim’s skin grows pale as their strength and vitality diminishes. The longer the sickness, the victim begins to suffer from an extreme chill that they cannot escape from. In extreme cases some victims have been found burned alive attempting to escape the cold. The infected wound becomes a deep black and blue-ish grey fluids seep from it, though no pain can be felt from it or around it.
For every 5 points of damage the victim takes from the disease, the creature suffers a 1 point deduction to STR and CON. The creature dies if one of its stats are reduced to 0.
If the victim takes 15 damage from the disease their infected wound becomes permanently black and occasionally emits a small wisp of smoke. The creature permanently gains the Wound of Darkness feat.
The disease and all stat reductions from the disease can be removed with a Greater Restoration. The disease naturally ends if the creature makes 3 consecutive successful saves or the creature takes a cumulative 25 points of damage.

Sequela

Wound of Darkness

After battling the darkness within, survivors of Shadow Sickness bear the mark of a darkened scar and tend to become withdrawn and emotionally isolated. 
Prerequisites: Character must have a permanent scar from a shadow sickness-infected wound. 
Benefits: The creature gains proficiency in Con saves. Once per long rest, you may use your action to draw on the remnant of darkness magic in you to gain darkvision out to 30ft for 1 hour. You also are immune to future cases of Shadow Sickness.

Prevention

Don't get wounded in magical darkness.

Type

Magical

Origin

Magical

Cycle

Short-term

Rarity

Extremely Rare