It's well-known that the Slaying God Spear was created by the Demon Ancestor Luo Hou himself. Therefore, it makes perfect sense that the Primordial Heavenly Demon Lord, as a descendant of the Demon Ancestor, could wield the power of the Slaying God Spear.
"Elder ancestor, tell me what you know of the Demon Ancestor's legacy, and then, I can pretend to be the Primordial Heavenly Demon Lord and use the pure techniques of the Demonic Way to rescue you," Jiang Chen said.
As for the Underworld River ancestor, Jiang Chen didn't have anything to hide and straightforwardly shared his plan to frame the Demonic Way, an undertaking that required his counterpart's cooperation.
"Once this deed is done, if anyone asks you, Elder Ancestor, who saved you, I need you to insist without hesitation that it was me who did it," Jiang Chen said.