“It's a condition that affects titan creations. Th' titans fashioned their creatures with stone, mostly, or other mechanical means, so they could carry out their assigned duties tae th' world without fear o' deterioratin' or gettin' weak. But there are beings o' great magic an' malice that hate th' titans, and they sabotaged these creations by turnin' their bodies tae flesh like th' other creatures o' Azeroth.”
The Curse of Flesh (also spelled the curse of flesh) is a curse created by the Old God Yogg-Saron suffered by many titanic creations. The Curse originated from the Forge of Wills and slowly spread through the titan-forged, turning them from metal and stone to flesh beings.
During the ordering of Azeroth, the titans of the Pantheon were unable to kill the Old Gods without completely destroying the planet, thus they were neutralized and contained within prisons. The titans left behind beings called keepers as jailers of the Old Gods' prisons. Thereafter, the keepers created machines to produce new titan-forged, one of those was the Forge of Wills.
A long time after, the Old God Yogg-Saron, imprisoned within Ulduar sensed a great disturbance in the keepers that were in charge of guarding his prison. This disturbance was caused by the death of the Pantheon. Yogg-Saron used this chance to weaken his jailers. By corrupting the Forge of Wills, tainting it with a malady known as the Curse of Flesh, any titan-forged being created by it would transform into a mortal being of skin and flesh, in order to "facilitate assimilation". The curse would even spread to the previous generation of titan-forged.
Yogg-Saron then proceeded to corrupt the keeper Loken, tricking him to create an army with the Forge of Wills and attack his former allies. He realized his mistake too late, as he noticed the dark affliction on his army. After Loken's betrayal many titan-forged were exiled, the first to be affected by the curse were the vrykul, whose metallic skin became brittle and weak. Generations later, the afflicted vrykul continue to degenerate into mortal beings later known as humans. Many other titan-forged - mechagnomes, tol'vir, mogu, and giants - would suffer a similar fate. However, Yogg-Saron's plan had unintended consequences, it gave rise to mortal qualities of necessity: courage, resolve, and heroism.
The keepers attempted to cure the curse and extirpate the symbiotic infection unsuccessfully, recording the history of the curse and their experimentation on the Discs of Norgannon, while telling how the Earthen suffered the effects of the curse. Keeper Ra lost faith when he discovered the titans of the Pantheon had vanished and Azeroth had been infested by the Void's Old Gods, and he secluded himself from society. Keeper Archaedas and his companion Ironaya spent too much time thinking on how to cure the curse, becoming obsessed with it, until eventually entering a long period of hibernation. Unable to cure the Curse of Flesh, mechagnomes and later the earthen would be afflicted by it.
During the war against the Lich King, Gearmaster Mechazod began curing gnomes of the curse of flesh and transforming them into mindless mechagnomes until he was defeated by adventurers. Years later, during the Fourth War, King Mechagon attempted a similar plan using his Mechoriginator device but failed.
- Earthen / Dwarves — The titans created the earthen to shape the deep reaches of the world. After Archaedas and his allies stole the Discs of Norgannon from Loken, they traveled south to Uldaman with some earthen and mechagnomes and hid the Discs there. The earthen, fearing the Curse of Flesh, asked to be put into hibernation until the day would come that a cure could be found. The mechagnomes, even knowing they were also cursed, volunteered to stay awake and keep watch over the earthen. 
- Stone troggs / Troggs — The stone troggs were the titans' first failed attempt to create earthen. When afflicted by the Curse, the stone troggs were turned into troggs, though stone troggs can still be found in Deepholm.
- Mechagnomes / Gnomes — The titans, or, more specifically, the titanic watcher Mimiron, created the mechagnomes as servants and caretakers, much like the earthen. When afflicted by the Curse, the robotic mechagnomes turned into fleshy gnomes over the course of thousands of years. A mechagnome by the name of Gearmaster Mechazod later found a way to reverse the effect, turning a large amount of unlucky gnomes in the Borean Tundra back into mechagnomes, before he was ultimately destroyed by adventurers. King Mechagon also intended to reverse the curse from the afflicted races.
- Iron Vrykul / Vrykul / Humans — When afflicted by the Curse, the newborn children of the vrykul of Northrend began to be born stunted and weak. In response, King Ymiron decreed that all malformed infants were to be killed at birth and that the vrykul would renounce their former gods who had now forsaken them. However, some vrykul parents secretly escorted their children to safety, providing them with limited supplies and leaving them on an unfamiliar shoreline in the land that would later become known as the Eastern Kingdoms. These malformed vrykul children would later become known as humans.
- Kvaldir — One vrykul tribe, in particular, was hit so hard by the malaise that it nearly consumed the tribe. The tribe sought the aid of its priestesses, who reached into the world of spirits for answers, but instead, they only found a malevolent entity lying in wait. The entity further corrupted the afflicted vrykul, who would forever after bear the name "kvaldir".
- Mogu — The mogu were initially stone beings created to help the titans carve the mountains and valleys of the land that would later become known as Pandaria. Even when their master, Ra-den, fell silent, the mogu continued their work, until they were hit by the Curse. Along with being turned to flesh, the mogu began to feel pride, greed, fear, and anger, and began to fight amongst themselves. Much later under Lei Shen, however, the mogu found a way to reverse the curse of flesh among themselves. /
- Tol'vir — The cat-like tol'vir were created to maintain titan lore repositories and machinery surrounding the titan cities of Ulduar and Uldum, though they were later afflicted by the Curse. Thousands of years later, the Neferset tribe of tol'vir allied with Siamat, a servant of Al'Akir, who in turn had allied with Deathwing, in order to restore their stone forms. /
- Giants — According to Brann Bronzebeard, most forms of giant got hit with the Curse of Flesh, but it doesn't appear to have affected them much.
- ^ a b Quest for Pandaria — Part 3, pg. 4
- ^ World of Warcraft: Chronicle Volume 1
- ^ History on common tol'vir archaeology artifacts
- ^ a b Ultimate Visual Guide, pg. 22
- ^ a b c d World of Warcraft: Chronicle Volume 1, pg. 54 - 56
- ^ a b c Tribunal of Ages
- ^ World of Warcraft: Chronicle Volume 1, pg. 60 - 61
- ^ World of Warcraft: Chronicle Volume 1, pg. 66
- ^ World of Warcraft: Exploring Azeroth: The Eastern Kingdoms, pg. 52
- ^ World of Warcraft: Chronicle Volume 1, pg. 81
- ^ World of Warcraft: Chronicle Volume 1, pg. 125
- ^ World of Warcraft: Chronicle Volume 1, pg. 137
- ^ World of Warcraft: Chronicle Volume 1, pg. 62 - 63
- ^ Mechagon
- ^ a b c Ask CDev
- ^ Agents of Order
- ^ The Curse and the Silence
- ^ World of Warcraft: Chronicle Volume 1, pg. 81
- ^ World of Warcraft: The Magazine Volume II Issue I