Just now on the road, the Demon King had already detailed the situation of the All Saints Dragon King.
Sun Wukong had only then learned that the title of the old dragon, Dragon King, was not bestowed by the Heavenly Court but self-proclaimed.
It turns out, the All Saints Dragon King and the North Sea Dragon Race are of the same lineage. When he could not inherit the position of the North Sea Dragon King, he went to the North Dark Lands.
Leveraging his relation with the North Sea Dragon Race, he then started doing business.
He sold some specialties of the North Dark Lands to various parts of the Three Realms and made considerable profit, thereby building good relations with all parties.
Such as the Demon King, who is a frequent client of the All Saints Dragon King— their relationship was quite good.
The reason All Saints Dragon King introduced the Nine-headed Worm must be due to this relationship.