Because he feared that burning Chu Feng to death might result in being unable to cure a poison if ingested, the Beixie Emperor dared not act rashly. Hence, the need to untie the bell by the bell-ringer himself.
He ordered not to use fire attacks, only allowing conventional methods to continue the strong assault.
In the Thunder Cave, Chu Feng and others put up a tough resistance, but that goes without saying.
...
Just after Chu Feng and his group left, a series of bizarre incidents occurred within the Ten Thousand Sword Sect.
One after another, sect disciples went missing.
Neither alive nor dead bodies were found.
The entire Ten Thousand Sword Sect was shrouded in panic. Rumors started spreading among the sect members, some saying that Chu Feng and his group had returned for revenge. Others suspected that spies from the Rashamon Sect had infiltrated the Ten Thousand Sword Sect.
In short, the Sect Elders investigated repeatedly and heightened their defenses to no avail.