If the ancient records are correct, the ancient mages who fell into the Spirit Realm and transformed into demons are nearly immortal, especially those from the faction known for their immortality.
Destroying the opponent's material vessel is merely severely injuring them. Given enough time, demons can still be reborn in the Abyss of the Spirit Realm, and the stronger the individual, the quicker their Demon Soul can recover.
Considering the degree of chaos in the spacetime of the Spirit Realm, this speed likely largely pertains to the demon's subjective perception of time.
Besides sealing and binding their essence, there truly isn't a better method at present to completely eliminate a Spirit Realm demon.
If you kill them and let them return in the future, I won't have the second chance to be bestowed with Star Soul Divine Power to combat them.
However, now it seems this powerful demon can still enter dreams and babble even if sealed, posing real danger.
