Rahula had already reached his limit in refining the Immortal Slaughtering Sword Formation to its current state.
Thus, even though Rahula knew that the Immortal-Severing Sword was the formation's sole flaw and weakness, he had no way to remedy it.
Of the Four Immortal Slaughtering Swords and the Immortal Slaughtering Formation Diagram, one had been forged after birth, while the other had been nurtured innately.
They did not share the same root and source to begin with, so fusing them perfectly was impossible.
At least, it was beyond the current Rahula.
Setting up the Immortal Slaughtering Sword Formation required fusing the Immortal Slaughtering Formation Diagram with the Four Immortal Slaughtering Swords, using the diagram as the formation base and the four swords as the formation eyes.
Among them, the Immortal-Severing Sword served as the connecting core between the four swords and the diagram—the core that bound the entire sword formation tightly together!
For that reason, the Immortal-Severing Sword could not serve purely as a sword of ultimate slaughter like the other three.
The Immortal-Severing Sword had to fulfill a certain supporting role.
That was what weakened its killing power.
Moreover, as the connecting core, the Immortal-Severing Sword should not really have served as a formation eye in one quadrant. It should have been tucked away out of sight instead.
Rahula had originally thought so too.
But in the end, he discovered that it would not work.
It was not that the Immortal-Severing Sword could not be hidden, nor that Rahula could not forge a fifth killing sword.
Rather, it was due to the restrictions of the Immortal Slaughtering Formation Diagram.
The Immortal Slaughtering Sword Formation used the Immortal Slaughtering Formation Diagram as its formation base.
That meant the entire sword formation had essentially been fixed from its very inception. Rahula