Except for Flood Dragons and other demon beasts with inherently powerful bloodlines, direct descendants of Tier Five demon beasts must undergo the Celestial Thunder Tribulation when advancing to Tier Five, using its power to purify and elevate their bloodlines.
The "Earth Dragon Worm," like the Golden-winged Thunder Eagle, is a high-tier demon beast that becomes a Tier Four demon beast upon reaching maturity. Its bloodline level is quite high, but to advance to Tier Five, it too must undergo the Celestial Thunder Tribulation.
Zhouyang is not unfamiliar with such demon beasts, because once, at his Foundation Establishment Cultivation stage, he was swallowed whole by an immature "Earth Dragon Worm," almost perishing in its stomach space filled with corrosive acidic fluid.
As fate would have it, the culprits who caused him to be swallowed by the "Earth Dragon Worm" were none other than the parents of the Golden-winged Thunder Eagle he now rides.
