For 20 whole years, Alex cultivated tediously, stopping not once to take a break. There wasn't much that needed his presence, so there was no need for him to stop. He simultaneously worked on learning more about the Dao of Radiance as well as slowly converting one of his Origins to form a shell so that it could become a Spirit.
In the 2 decades, he had improved his cultivation by quite a bit, to a point where he was much closer to reaching the Immortal Spirit realm, and thus understood one very important thing.
He did not require another Spirit in order to break through at all. The Dragon Father's eye had been all that was needed of him. That was most likely because he needed his Soul to have undergone some form of transformation before he could reach the Immortal Spirit realm, and the Dragon Father's eye fulfilled that requirement.
