Flesh Cancer was the byproduct of an experiment during the Wizard World's Enlightenment Period, conducted by a Sixth Circle Great Wizard in a bid to breach the Seventh Circle.
But the result was, undoubtedly, a failure.
He created a mass of ever-growing twisted flesh that, upon its birth, insanely drew in energy from its surroundings like cancer cells and multiplied uncontrollably.
For some reason, that Sixth Circle Great Wizard, in a fit of madness, fused this twisted flesh into his own body.
As a result, his soul, along with his body, was consumed by the mass of flesh, thereby fermenting a notorious disaster of the Enlightenment Period—the Carnage of Flesh.
Thousands of Wizards of varying strengths, as well as tens of millions of mortals, fell victim to it.
In the end, this twisted mass of flesh was sealed by several Sixth Circle Great Wizards, and it came to be known as Flesh Cancer.