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 Plant. This is the dwelling place of Xurkitree.
In order to battle Xurkitree, ascend through the area that is actually shaped like a Xurkitree. By reaching the top, you wil find and be able to battle Xurkitree. Xurkitree remains a pure Electric Type Pokemon, always being found at Level 60 and knowing the moves of Hypnosis, Discharge, Electric Terrain, and Power Whip. 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.