Gloom is an enemy substance and obstacle in Tears of the Kingdom.
Tears of the Kingdom
"The gloom that covers the ground cannot be dispersed—not by a weapon, nor by fire, water, wind, nor any other method"
Link and Zelda traveled Beneath Hyrule Castle in search of the source of the mysterious "gloom" substance - named by Purah - afflicting citizens of Hyrule, and causing them to fall ill. There, they found a corpse-like Ganondorf radiating the substance, despite being pinned down by a strange arm. As they approached, a tear-shaped object fell from the hand, allowing Ganondorf to revive. He then attacks Link with a concentrated burst of gloom. This ravaged Link's right arm, shattered the Master Sword and destroyed both 27 of Link's 30 Heart Containers & two of his three Stamina Wheels, reducing him to the strength he had when he woke in the Shrine of Resurrection. Ganondorf then used the gloom to trigger the Upheaval, opening Chasms to the Depths across Hyrule, raising Hyrule Castle into the air and many other effects. Zelda fell into the first of these chasms, but as Link dove after her, she vanished in a burst of light. He was then rescued and taken to Great Sky Island by the mysterious arm, and had his destroyed right arm replaced with it.
When he subsequently returned to the surface, he found many chasms open, all infested with gloom and making many people sick; although many recovered on exposure to sunlight, some - such as Claree and Lasli's Nanna - suffered a more severe Gloom-Borne Illness, leaving them bedridden.
When Link is exposed to gloom after Ganondorf's initial attack, it temporarily disables heart containers. This works from his rightmost heart container regardless of his current health. If he runs out of heart containers, he dies.
On the surface, this effect lasts only as long as he is being continually exposed to gloom - although when heart containers are re-enabled, he still needs to heal as normal to fully restore his health. In the Depths, however, the effect does not automatically reverse in whole or in part unless he travels to an active Lightroot, eats a "Sunny" meal or is revived by a fairy.
If Link has multiple Temporary Hearts, they are treated as a single Heart Container for the purposes of gloom damage. As with any temporary hearts, they disappear permanently after loss.
In addition to static gloom lying on the ground, many enemies in the Depths are "Gloom-Wreathed", and will deal gloom damage - disabling heart containers - rather than ordinary damage.
There are also active pools found in the Depths, Labyrinths and, occasionally, other areas such as Akkala Citadel Ruins. These chase Link and form Gloom Spawn, giant hands with eyeballs, to grab him. If Link escapes these, they evaporate and leave Dark Clumps behind. If he confronts them - as is required under the Deku Tree to free Korok Forest - actively defeating them spawns a Phantom Ganon.
Gloom-based substances cause Regional Phenomena for certain races. The Gorons' tough bodies seem to resist Gloomy mortality, so Ganondorf disabled them through other means. Marbled Gohma, his minion, infects Death Mountain with Gloom, resulting in the creation of Marbled Rock Roast, an extremely addictive narcotic that entirely neuters the Goron working force. Children and the elderly can't chew through the tough substance, but they alone are unable to keep the YunoboCo gem-mining operations afloat.
The Zora are also affected by Gloom, this time taking the form of Sludge. This watered-down version is too weak to harm Link directly, but reduces his movement speed significantly when he attempts to wade through it. However, as Muzu later admits, the Zora are noticeably susceptible to Gloom, and rely on water to survive, so the Sludge in the domain proves life-threatening for them, with King Dorephan himself nearly dying from it. Copious quantities of Sludge source from Mucktorok, spitting it down from the Sky-based Water Temple; more still is also produced by the Sludge Like, the monster who almost killed the Zora monarch.