https://serebii.net/scarletviolet/teraraidbattles/event-shinyrayquazaappears.shtml
Based on the black Rayquaza from the Pokemon Horizons anime. It's only a 5-star raid so it's fairly solo-able if you have a good Fairy-type Pokemon built for raids like Azumarill or Sylveon. You can do the raid as many times as you want for extra rewards, but you can only catch the Rayquaza once per save file. There's also a bunch of 5-star Blissey raids as well, for easy Exp candies.
EDIT:
Only one Rayquaza raid will spawn in your game per day, but if you really want to farm them (apparently they're great for herba mystica) then fiddling with your system clock does work. Just go to your system settings without exiting your game, open the option to change your clock, set the clock one minute backwards, and that will refresh your current outbreaks and raid dens.
https://i.imgur.com/PmF1p9q.jpg