To get the REGI pokemon, you must know the key. To know the key, you need to understand the game's Braelle (I murdered the spelling... T^T) but to understand that, you need a Braelle Key in real life, or understand Braelle. Then, you need to go into the cave, but that needs a key too, so you need to perform a specific action as the key. Then you must fight the pokemon and catch it.
BUT WAIT!
Regigigas was actually the REGI GOLEM, and the other REGI pokemon who DON'T LIVE IN THE SAME REGION are the key. Somehow, somebody who lives 3 years in the past knows that he can transfer their three legendary pokemon to some unknown region, 3 years in the future, as a key to unlock the golem.
So, basically, to get Regigigas, you need a key for a key, for a key, for a key, to get one of three keys, and THEN you need to have to be able to time-travel and see where the keys are meant to go. That makes perfect sense. Gotta love 'dem REGI'S!!!
BUT WAIT!
Regigigas was actually the REGI GOLEM, and the other REGI pokemon who DON'T LIVE IN THE SAME REGION are the key. Somehow, somebody who lives 3 years in the past knows that he can transfer their three legendary pokemon to some unknown region, 3 years in the future, as a key to unlock the golem.
So, basically, to get Regigigas, you need a key for a key, for a key, for a key, to get one of three keys, and THEN you need to have to be able to time-travel and see where the keys are meant to go. That makes perfect sense. Gotta love 'dem REGI'S!!!