First off here is the code that you need to unlock the Mountain Door key, Steel Doot Key and the Iceberg Door Key This will be for people using Desume 0.9.6+ or if you have action replay This will unlock all the keys since without this the only other way would be to Trade keys with someone else who has the opposite key
Code To Unlock All Regi Door Keys:
I also Did a commentary over this to make it much easier as other people put text which is hard to follow.. So If this has been a major help please leave a like and subscribe for more in depth videos on catching legendarys.
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MORE INFO ON THE REGI TRIO AND REGIGIGAS:
Regirock (レジロック-) Deep in the Clay Road are the Underground Ruins. These ruins are seldom seen caves which hold a massive puzzle. The first one is the focus of the Pokémon Regirock and has you have to do a puzzle that can only be known if you visit the place at Day and night. The puzzle requires you to move 6 steps down from the centerpiece and then move 9 steps right. Then select down to hit a hidden switch. This changes the ruins so Regirock becomes available.
Regice(レジアイス) - Following on from the capture of Regirock, you will receive a special Unova Link key. The Steel Key in Black 2 or the Iceberg Key in White 2. When you switch over to using these via the Unova Link function, you will then find that they instead have taken residence in the same place you captured Regirock.Typically, this would make them version exclusives with Registeel being Black 2 and Regice being White 2. However, the ability to share across Unova Link means that if your friend has the opposite game and has captured Regirock and got the key, he can send that key to you.If you are Playing Pokemon Black 2 you need to Transfer Iceberg key from Pokemon White 2 or Trade Regice from a friend.
Registeel (レジスチル)- If you are Playing Pokemon White 2 you need to Transfer Steel key from Pokemon White 2 or Trade Resteel from a friend.I traded Steel Key from my Pokemon White 2.
Regigigas (レジギガス) - Once you have captured Regirock, Regice and Registeel, you now will be able to access the Colossal Pokémon, Regigigas. If you bring the three Pokémon with you to where Regigigas sleeps in the basement of Twist Mountain, and interact with it, it will wake up!
Regigigas (Both, You need the other three)