To be specific, Atherton did know that the blood clan would have to go to war against the celestial for the second time after a thousand years, but he didn't know when the celestials would attack.
Because of this, he asked the elders to develop the crimson barrier, not only for the future of his kingdom but also for this war that he had seen in his vision.
As for resurrecting the blood clan ancestors...
Atherton felt that if he could do it when he opened his domain within a few minutes, why should he let the two elders waste a thousand years just to revive a few ancestors?
At most, they could only revive ten people!
The celestial ancestors didn't know that inside Atherton's domain, he could revive these ancestors at their peak strength, as long as the ancestors never left his domain.
Once they left his domain, they would all turn back into ashes, leaving behind only a drop of blood that was left way before they first died.
