George R.r. Martin said this quote
“ His long wait is almost done. I am sending Balon Swann to Sunspear, to deliver him the head of Gregor Clegane.” Ser Balon would have another task as well, but that part was best left unsaid.“Ah.” Ser Harys Swyft fumbled at his funny little beard with thumb and forefinger. “He is dead then? Ser Gregor?”“I would think so, my lord,” Aurane Waters said dryly. “I am told that removing the head from the body is often mortal. ”