The Mutalith Vortex Beast is less a creature than a wound in reality given monstrous shape, a being crafted or grown to serve as a vessel for the untrammelled power of change. Its hulking, ever-shifting body is crowned with a swirling vortex of warp-energy, a maelstrom that leaks the raw stuff of transformation into the world around it.
On the battlefield its presence alone is a weapon. The energies that pour from its vortex wreak havoc upon everything nearby, causing enemy soldiers to sprout hideous mutations, dissolving equipment into slag, and sowing terror and madness among those who witness its approach. When goaded to unleash its power deliberately, it can call down cascades of arcane destruction, reshaping the very ground and reducing formations of the enemy to writhing ruin.
For all its power the Mutalith is a volatile and half-controlled thing, and the sorcerers who direct it do so at a careful remove, wary of the untamed forces it embodies. It is, in essence, the flesh-change made monstrous and turned into a weapon of war, a shambling testament to the Thousand Sons' terrible mastery over the powers that both serve and threaten them. Where it walks, the laws of nature falter, and nothing that stands against it remains unchanged.