Decay, sometimes referred to as the "sixth element", is a primordial force. 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 shaman 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 Thros as a realm of death, decay, and nightmare.