True Phoenix!
In the legends, True Phoenixes were on par with True Dragons, and True Dragons were the strongest of divine beasts. Nine Dragons Tyrant Body Art, which Ling Han cultivated, used nine dragons as a power standard, and it was precisely because the power of dragons was the strongest.
Divine beasts, blessed by heaven, didn't need to cultivate at all. At maturity, they possessed terrifying strength, and the power of True Dragons could even shatter the firmament, breaking open the universe, and it became god realm's measurement standard for power.
Of course, martial artists cultivating could still surpass the power of nine dragons, or else there wouldn't be the saying of the power of nine dragons.
But here, the power of True Dragons and True Phoenixes was definitely the limit of the extreme. If Azure Phoenix Divine Empress could truly possess the complete power of a True Phoenix, then with an arbitrary slap, this Corpse King would disperse into a pile of bones again.