Firstly, he sent Murong Longcheng, Giant Titan, Poison Python, and Ling Wu into the Time Hall. He helped them adjust the flow of time before he entered himself.
Li Xiandao didn't choose 1,000 years, and instead, he burned 2,000 years straight away.
Burning 2,000 years allowed him to spend 1,000 years in the Time Hall. Li Xiandao stepped in, closed his eyes, and slowly improved himself.
Li Xiandao also brought the Soul Suppressing Stone in. It exploded with a loud boom. It released all the soul power that was contained within.
The energy surged!
A black ocean spread throughout the Time Hall.
Fortunately, Li Xiandao was in a separate space that was huge enough, pretty much an unlimited amount of space. It was able to contain this whole black ocean.
Li Xiandao didn't sit cross-legged. He just laid there and was drowned in the black ocean.