After defeating Ultra Necrozma and returning to The Alola Region, the Ultra Recon Squad will continue to allow you to explore Ultra Space with the help of their Legendary Pokemon. While traveling through Ultra Space, you may sometimes enter a White Wormhole that leads to Ultra Crater. This is the dwelling place of Celesteela.
In order to battle Celesteela, you will have to make it past all of the exploding craters. At the very end will be Celesteela, rising out of a crater of its own. Celesteela remains a Steel / Flying Type Pokemon, always being found at Level 60 and knowing the moves of Autotomize, Seed Bomb, Skull Bash, and Iron Defense. Like all other Ultra Beasts, it can now be found in its Shiny Form. It will also respawn with each successive visit to this area.