The Flesh Change was the terrible affliction that gnawed at the gene-seed of the Thousand Sons, a mutating sickness that warped a warrior's body into a shifting horror of bone, sinew, and errant flesh. It struck without warning, unravelling the careful genetic work of the primarch Magnus and reducing proud legionaries to raving, misshapen ruins.
For long years the Legion dwindled toward extinction as the curse spread through its ranks, until Magnus the Red wrought a sorcerous working that seemed to halt the decay and restore his sons. Yet the salvation carried a hidden price, binding the Legion ever more deeply to the practice of psychic power and the perilous currents of the warp from which such power flowed.
The Flesh Change was never truly cured, only held at bay, a lurking dread that shaped the Thousand Sons' obsession with knowledge and control. In the ruin that followed the Council of Nikaea's censure, the affliction would return to haunt them once more.