Curse of Flesh

"'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.'""— Felyae Ironleaf, historian and archaeologist of the Explorer's League specialized in Titanic creations"The Curse of Flesh is a curse created by the Old Gods inflicted upon many titanic creations. The Curse slowly transformed the early races of Azeroth from metal and stone into skin, blood, and bone, weakening them in the process.

History
During the Ordering of Azeroth, the Titan Pantheon was unable to permanently destroy the Old Gods without destroying the planet, thus they were sealed within the core of the planet and imprisoned. To ensure that the Old Gods would not break their seals and return to decimate the world, the titans left behind beings known as keepers as jailers of the Old Gods' prisons. Thereafter, the titans created machines to produce new proto-beings, one of those tools was the Forge of Wills.

Millennia 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 Sargeras' destruction of the Titan Pantheon. With their last strength, the titans infused their energies within the keepers. Yogg-Saron took advantage of this chance to weaken his jailers and eventually escape. By corrupting the Forge of Wills, tainting it with a malady known as the Curse of Flesh, any titan-forged being created by it will fall victim to the infection, and it will even spread to the previous generation. The Curse of Flesh will transform the titan-forged into mortal beings of skin and flesh, in order to "facilitate assimilation".

Yogg-Saron then proceeded to trick the keeper Loken into creating an army with the Forge of Wills to attack his former allies. Loken 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 of which affected by the curse were the vrykul, whose metallic skin became brittle and weak. Generations later, the afflicted vrykuls 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 necessary mortal qualities such as courage, resolve, and heroism.

The keepers attempted to cure the curse and extirpate the symbiotic infection unsuccessfully. Keeper Ra lost faith when he fount out that the Pantheon had vanished and Azeroth had been infested by the Void's Old Gods, he secluded himself from society until his eventual murder. Keeper Archaedas and his companion Ironaya spent so much time thinking on how to cure the curse that they became 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 the it.