Hidden Grottos Missing in API

[notblisy]

This encounter type is exclusive to B2W2:

• https://bulbapedia.bulbagarden.net/wiki/Hidden_Grotto#Locations

I think this is fairly simple to include. We add a new encounter type called "hidden grotto"

After that, it's a matter of adding encounters to each location_area and encounter slots.

I don't think much else needs to be added for them.

I'll look into this in the coming weeks if no one else does as I chip away at missing encounter data in gens 1-7.