Gu An did not directly annihilate the Great Immeasurable Devil Ancestor. He was searching through the Great Immeasurable Devil Ancestor's memories and karma.
The Dao Universe the Great Immeasurable Devil Ancestor had opened up was not something he had imagined himself. In the early days after he became a Saint, he had once gone to a mysterious place to listen to the Dao. There, he had become acquainted with the Dao Lord, and there he had also conceived the idea of a Dao Universe.
In any case, his Dao Universe had been modeled after another. In other words, other Earths still existed.
As a Saint, the Great Immeasurable Devil Ancestor's memories were unimaginably vast, weightier than all the years and history of the entire Heavenly Dao combined.
Even for a Saint, digesting another Saint's memories required time and effort, but Gu An surpassed Saints. He was a Wuji Super-Origin Venerable Saint, and absorbing a Saint's memories was no difficult matter.
Gu An had only sat for a short while before he successfully digested the Great Immeasurable Devil Ancestor's memories.
He then ground away the Great Immeasurable Devil Ancestor's saint soul, reducing him to flying ash and scattering smoke.
[You successfully seized 4,903,344,284,000,000,000,409,552 years of lifespan from the Great Immeasurable Devil Ancestor (late-stage Primordial Taiyi True Saint Realm)]
Looking at the prompt before him, Gu An's state of mind did not change at all. After all, this was not his first time killing a Saint.
As for lifespan, the lifespan seized this time was still not enough for him to break through, so naturally he was not too pleasantly surprised.
Through the Great Immeasurable Devil Ancestor's memories, Gu An gained a certain understanding of that Dao Lord.
This group of Saints centered around the Dao Lord, attacking and plundering Dao Universes