Zhan shi and the others might not have been aware of the secret hidden within Sage Kui's sculpture, but Zhang Xuan was well aware of it.
The upper body of Vicious, the formidable Otherworldly Demon who had fought on equal grounds with Kong shi back in his prime, was sealed within the sculpture!
Back then, after he found that Vicious' upper body was stored in the sculpture with the Library of Heaven's Path, he had intended to smash that sculpture into bits to retrieve the upper body. But in the end, he had forced himself to calm down and decided against it.
No matter what, he was still a master teacher and the head of the Sanctum of Sages. Even if it was to achieve his goals, he had to take note of how he went around doing it.
In any case, for the Otherworldly Demons to barge into the Inner Sanctum and take only Sage Kui's sculpture, there was no doubt that it had something to do with Vicious' upper body!