Monster I'm working on called "Thisf*ckinthing"

backstory: 
Nobody knows what Thisfuckinthing was originally supposed to be.The oldest surviving account comes from a goblin chronicler who only wrote one hastily scribbled sentence on a blood soaked page in his journal:
“THIS FUCKIN THING CAME OUT OF THE HOLE.”
The pages immediately afterward are missing, torn out, the remainder being blood stained with no clear cause.

Long ago, in the chaotic fringes of the Lower Planes where magic and anatomy warp under raw malice, a cabal of deranged archmages and flesh-shapers sought to forge the ultimate vanguard for purposes unclear. Some who know the stories say they wanted thebuktimate beast of burden that defied not only steel,spell, and exhaustion but the fundamental laws of anatomy and logic itself. Others say they wanted a weapon of war that could terrify the enemy and harness the regenerative capabilities of trolls.
The origin of the creature traces back to a catastrophic ritual gone awry. The mages attempted to fuse the physical essence of a wolf, a troll and a twisted aberration, but an arrogant apprentice botched the binding circles at the final moment. The energies inverted upon themselves, twisting the entity's core anatomy inside out. Its skull was violently relocated to the base of its spine, and its limbs were cursed with a stubborn, horrific autonomy that refused to accept death.
When the containment cells inevitably failed, the creature slaughtered its creators not through complex strategy, but through sheer, disorienting wrongness. Warriors who faced it in the dark found their minds fracturing long before their flesh, unable to track which way the beast was facing or where its strikes were coming from. Now, these abominations wander the desolate wastes and subterranean dark. They hunt by ambush, contorting their grotesque frames into stillness, even on vertical surfaces or upside down on cave ceilings, lying in wait until an unsuspecting traveler mistakes them for a buried corpse or a bizarre rock formation—only to realize too late that every assumption about its anatomy is a fatal mistake.