Gloom

is a substance in.

Characteristics
TotK: is very similar to Series:, and is produced by TotK:. When TotK: comes into contact with it, it temporarily reduces his maximum hearts. Hearts that have been depleted by Gloom will appear with the last N hearts being gray and broken, allowing them to be distinguished from damage taken from other sources. The amount of hearts reduced is always whole hearts (meaning maximum cannot be reduced by quarters of a heart). If Link have any temporary hearts prior, any gloom damage will destroy all of those hearts first without any regular hearts becoming grey and broken.

TotK: saps the strength of creatures upon touching them, in some cases leaving them sluggish and unable to move. Being on the TotK: or the TotK: allows TotK: to recover from losing hearts to TotK: at a rate of 1 heart per second. However, being in the TotK:, being spotted by Gloom Spawn or if Phantom Ganon is present, prevents TotK: damage from healing over time, unless TotK: is standing beneath a TotK: or eats TotK: made with a TotK: or TotK:. Sleeping on a comfy bed restores hearts lost by gloom and also fills them.

NaN Enemies in the TotK: are frequently wreathed in TotK:, and will similarly reduce TotK:'s maximum health when attacking him. Their gloom damage is calculated:

$$MaxHeartsLost = Ceiling(Ceiling(Damage * 0.3)/4)$$

Where damage is the amount of damage (in quarter hearts) that their non-gloom-wreathed equivalents do. For example, getting hit by the punch attack of a Silver Bokoblin (when having no weapons) would deal 24 damage (6 whole hearts) to an armorless Link, but if that was Gloom-wreathed, it would deal 2 hearts of gloom damage (24 damage * 0.3 = 7.2, rounded up makes 8, divide that by 4 (round up again if there's a decimal) and that is 2, which is the amount of maximum hearts lost).

It should be noted that gloom damage will not reduce Link's filled hearts unless gloom damage reduces the maximum hearts below the amount of filled hearts he has. Thus if the only type of damage Link receives is gloom from enemies and hazards, filling the hearts is unnecessary.