Tu Wa Zi and Gu Chen naturally knew Stone Toughness.
In the past, while still in the Lower Realm, they had dealt with him a few times. Although their interactions were not deep, they knew of each other as a terrifying peer with an overwhelming presence.
Especially, what left the deepest impression on Tu Wa Zi and Gu Chen was the life-and-death battle that took place on the heavenly arena in the Void God Realm between Stone Toughness and Shi Hao.
In that battle, the two were equally matched, fighting to exhaustion, bloodied all over, using every single hidden ace they had. Shi Hao, whose strength was almost on par with them, was nearly shattered in body. Only by giving it his all did he manage to win by a hair's breadth, defeating the Double Pupils with the Soul Extinguishing Needle, and claiming a hard-fought victory.
They witnessed the entire battle from start to finish and questioned themselves: if they were to replace him, victory or defeat would be an uncertain outcome.