Actually, there is one more thing you’ll need these Regi for. First, you’ll need to somehow obtain (or borrow) the Regi that you didn’t catch in Split-Decision Ruins. Once you’ve gathered all 5 Regi (Registeel, Regirock, Regice, Regieleki and Regidrago), look for a peculiar max raid den in Giant’s Bed, near the entrance to Snowslide Slope.
This den is surrounded by four small rocks. If you interact with it normally, you’ll be told there’s nothing in the den–and you can’t use a Wishing Piece to activate it either. Instead, put the 5 Regi in your party. You’ll probably want your lead Pokémon to be a strong one as well–good picks include Zacian/Zamazenta or Urshifu.
Once you’re prepared, inspect the mysterious den with your party of Regi (and their plus one). If done correctly, the den will light up; go and have a proper look. Whoosh, you’ll be sucked into a special Max Raid battle!
Max Raid Battle
This hulking fella is the leader of the Regi. Although it can seem intimidating, it’s pretty easy if you act fast. You see, Regigigas has an Ability that halves its Attack and Speed stats for 5 turns. Unless you’re severely underpowered, the fight shouldn’t last longer than 5 turns.
For attacks, it can use Crush Grip, Zen Headbutt, Hammer Arm and Giga Impact. Most likely, unless you’ve trained the Regi, your main damage dealer will be your lead Pokémon. Fighting-types will do the most damage, but be wary of Zen Headbutt. Urshifu (Single Strike Style) or Pangoro needn’t worry about this though.
So long as you pick your Pokémon carefully and don’t let this fight drag on, you should win with just one Pokémon. If you made a mistake and your lead Pokémon faints, switch in Regieleki and use Thunder Cage to whittle its remaining HP. Unlike the other Regi, Regieleki isn’t slow as a mollusc so you can actually get some good damage in.
Bring Regigigas down to 0 HP and you’ll be able to catch it. It has a guaranteed catch rate too; easy!