Death and Decay is a signature spell of both the Death Knights of Warcraft II and the Liches of Warcraft III. It is a necromancy spell. The baleful energies of the spell cause anything caught inside to rot, decompose, and wither within seconds - even causing stone to blacken and crumble to dust. While Teron Gorefiend has possession of your body in Shadowmoon Valley you can cast this spell against his jailer Karsius the Ancient Watcher, and some mobs in Scourge-controlled areas such as Razorfen Downs utilize this spell.
The aphotic, swirling clouds conjured by the Death Knights can cause anything within their path to rot and decompose. The vapor created by Decay can consume anything - flesh, bone, wood or even the strongest metal. Heavily laden with base substances, these clouds descend and quickly diffuse, leaving only suffering in their wake.
The swirling negative energies known as the Death And Decay spell are favored by the liches for their power to decompose, rot and destroy everything within them. Not even the strongest warriors or the most durable structures can stand the full effects of Death And Decay.
Negative energy infuses the area in visible puffs. Living creatures age rapidly, as skin sags and flesh sloughs off bones. Structures wither and collapse, suffering the effects of decades within a few seconds. All living creatures and non-natural structures in the area take damage from the negative energy. Since undead are powered by negative energy, this spell cures them a like amount of damage.
In World of Warcraft
In-game, the spell is used by the mobs:
- Death's Head Cultists, quilboar casters outside the Razorfen Downs
- Sunseeker Gene-Splicers in the Botanica, Tempest Keep
- Rage Winterchill, a lich boss in Battle for Mount Hyjal
- Lady Deathwhisper, the second boss in Icecrown Citadel (instance). Hers has a greensih tint, making it distinct from the DK's version, which is red.
See this subject on the Hearthstone wiki: Death and Decay