Which area of the map is the den at, which den is it, and do I need to do anything specific besides catching the 5 Regis (like thrown in a Wishing Piece or talk to someone) in order for Regigigas to appear in the raid den?
Go to the Giant's Bed area of the Crown Tundra and find a Pokemon Den surrounded by four boulders, pictured below. It should never be glowing red. You'll need to have all five Regis in your party for the battle to activate, meaning you'll need to trade for whichever Regi you didn't choose at the Split-Decision Ruins.
How to Get Regigigas (IGN)
