Literature
Sonic and Opal Alola headcanon
Because of the Phantom Ruby's abilities and the scarring that was left behind when it was destroyed, more Ultra Wormholes are opening up in the Pokemon world. Solgaleo and Lunala already have their appendages full with their duties of maintaining the dimensions and keeping Ultra Beasts out, so they feel that they need aid in the form of a new Solgaleo or Lunala.
They can't create a Cosmog in this situation, however.
The two members of the Light trio turn to the Tapu for help, to which they decide to give the Cosmog to a trial goer to protect. This doesn't go as planned, for it is found by Lillie instead. Plan B involves a trial goer protect