Although he failed to track down the demonic cultivator he had originally intended to follow, the head of Black Nine that he brought back was worth far more than that cultivator.
Because Black Nine had not had time to release his Nascent Soul, the Nascent Soul was still intact.
The Kaiyuan Sect Supreme Elder easily extracted the purpose of the demonic cultivators' gathering through Soul Searching: they were gathering Demon Seeds in an attempt to restore the hegemony of the ancient demonic cultivators.
Chapter 19