C4501 36 Heavenly Astral Arts

This wasn't because the Unfailing didn't have the strength to kill Buddha Great Nirvana and the others. It was because they couldn't kill Buddha Great Nirvana and the others.

More or less, Buddha Great Nirvana and the others all had a peak Void Immortal Heavenly Venerate backing them up, or perhaps a True Immortal of the past who had once been a True Immortal of the Nine Heavens