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Demons, immortal and indestructible, difficult to eliminate and remove.
This has been a common understanding since ancient times, shared by immortals of old, to Corpse Resolution Cultivators, through to today's cultivators.
Heavenly Demons cannot be killed nor extinguished; they can only be sealed, a temporary solution to a permanent affliction!
Why is it a temporary solution?
Because as time passes, seals will eventually fail, and ultimately, the calamity of demons will leak out and wreak havoc once more.
It is said that time wears away all, and the years are unsympathetic; even the ancient immortals cannot live forever, and the immortal artifacts and spiritual treasures that have been passed down to this day are all damaged and incomplete, with reduced tiers and greatly diminished power.
However, demons remain demons, always indestructible!
Why?
Because of their origin, because of... their backing!