Although the Defy Ancient Sovereign Life Soul could provide Zhou Wen with strength that exceeded Slaughterer, the Defy Ancient Sovereign's strength was too potent. It was suited for frontal combat. Zhou Wen wasn't qualified to fight the black dragons head-on and could only use tricks. Therefore, Zhou Wen chose to use the Slaughterer Life Soul.
The strength increase provided by Slaughterer was inferior to the Defy Ancient Sovereign, but its characteristic of infinite Essence Energy was what Zhou Wen needed the most.
When the nine black dragons approached, Zhou Wen chose a location at the bottom of the sea that had a complicated terrain. Relying on the cover of the sea mountain, the black dragons didn't attack him immediately.