It was known for a while that Rillaboom would be the League Promo for Sword & Shield Season 1, and Frosmoth as the League Challenge promo. We now have images to confirm how they look and this will be the Voltage Beat version of Rillaboom. These also contain the new Reverse Holo pattern for this Generation.
This Rillaboom promo should be regarded as the better version of the two Rillaboom cards in Sword & Shield.
If you like these cards, you should visit your local Pokémon League soon to obtain these cards!
Thank you Eric Correa for showing the images!