The bones used in this formation came from a strong breed of beast in the Divinity. After death, the beast's bones were refined by a mighty warrior in the Divinity and were used in building a strong formation for defensive purposes.
Yan Tiangang's original plan was to get the bones for his own use. Later, he acquired a better Relic. So, he sent the bones and Divine Imperial City down at a huge cost.
Over the course of history, the bones helped disciples of the Yan Family over many disasters. Though the energy in the bones was almost used up, they had made yet another contribution.
During the three days, the one-hundred Stone Golems and the immortal's body never stopped blasting at the formation. Even the shield of the Demon-slaying City would have been ripped open under such force. However, there was zero indication that this formation was going to break.
