Great question! Ultra beasts only spawn in the Ultra Space Dimension, which can only be accessed when an Ultra Space portal spawns. Ultra Space portals naturally spawn in the world (often over water masses) and can be spawned in by certain legendary pokemon (i.e. Palkia, Dialga, Solgaleo). Once in the Ultra Space dimension, there will be a higher chance to encounter Boss Pokemon, Shiny Pokemon, and a possibility to find an Ultra Beast. There is no set timer for Ultra Beast spawns; however, you can check the latest Ultra Beast spawn by using /luand see if any Ultra Beast spawned in by using /ub. For more information regarding Ultra Space and Ultra Beast, please read through the Pixelmon Wiki page regarding the subject in the link provided below:
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