The explosion was a double explosion. The first one was the explosion created by the destruction of the secret realm, and the second one was from the deepest part of the secret realm. While the first explosion was not that horrible, the second one was the terrible one.
The destruction of the secret realm created an implosion, just like how something would be sucked into a black hole, thus creating a less impactful shockwave.
But the second explosion was created as the result of the power left behind by Kui Zheng after his death, and was triggered by the first explosion!
This explosion was whatever strength was left behind by Kui Zheng!
Obviously, this celestial race Qi Practitioner held extreme hatred towards Swords Gate after he was imprisoned by them. As such, he sealed his strength into his secret realm, making it easy to enter his secret realm, but hard to get out. The only way out was to destroy the secret realm.
