Decay is sometimes referred to as the "sixth element". It can be seen as the opposite of Spirit, the fifth element.
Unlike Spirit, which is used to communicate with the elements and/or heal, Decay binds the elements to one's will, forces them into servitude and weaponize them. This is also the main reason why dark shamans are able to use the elements (in a twisted form nonetheless) even without asking the elements for permission.
The Shadowlands are described as nightmarish realms of decay, and the Emerald Nightmare as a realm of death, decay, and nightmare.