Because these great prohibitions contain the indestructible will of ancient strong ones, even if they perished both in spirit and form in that distant past, the prohibitions they left behind are enough to plunge future generations into a deadly calamity.
Eternal Universe, Three Great Eras!
These Three Great Eras are hailed as the strongest, future generations can only speculate on how grand they were, but cannot truly comprehend the extent of the terror of the strong ones of these eras.
It's like a frog confined forever in a pond, the frog only sees the scenery of this limited pond, without knowing the vastness of the world outside it.
"Ah!"
Poof!
A beam of ancient divine light pierced through the abdomen of the Undying Heavenly Lord, the gaping hole bled profusely, subjecting him to an unimaginable calamity, a wretched wail escaping his mouth.
What of taboos?
What if they're dead?
