Yu Xian replied, "In all likelihood, it is related to the Demon Race, and possibly to a particular Demon Sovereign. Great Sun True Monarch must have obtained something from that Demon Sovereign, which allowed him to gain access to the fifth-rank array.
Also, I suspect that the Nine Yang Immortal Sect has a role in this matter.
The Sect Protection Array of the Profound Sun Sect was the first line of defense built by the Divinity Transformation Sovereign of the Nine Yang Immortal Sect to counter the Demon Race. The Nine Yang Immortal Sect holds the highest authority over the array.
Great Sun True Monarch being able to bypass the Sect Leader the True Monarch may not be without their influence.
However, the core of all this is the Green Phoenix Demon Venerate.
Whether it's the other Demon Sovereigns of the Demon Race, the Nine Yang Immortal Sect, or the Profound Sun Sect, they all can be directly or indirectly connected to the Green Phoenix Demon Venerate.