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 Forest. This is the dwelling place of Kartana.
In order to battle Kartana, you will have to make it to the end of the small forest path. However, on your way to it, you will actually have to battle a few trainers that battle using Kartana. Kartana remains a Grass / Steel Type Pokemon, always being found at Level 60 and knowing the moves of Leaf Blade, X-Scissor, Detect, and Air Slash. 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.