They found Karkaroff's body in a shack with the Dark Mark over it.
It was bitterly cold, so the smell wasn't yet bothersome. Nor was the corpse's condition that unsettling; Karkaroff had been kept surprisingly whole. Bones were broken, flesh was torn, frozen blood coated everything – but he was easily identifiable, and technically in one piece.
No, it was something else that made the sight disturbing. It was his telling expression: eyes squeezed closed, jaw tight, teeth and tongue stained crimson.
Torture had so broken him that he had died not with an agonized scream, but an animal's dumb whimper.