There are a lot of Key Items in this Pokémon game, here is a list of the most useful and hard to get Key Items in the game. This tells you what they do and how to get them.
How to unlock
Allows you to be able to Battle and Catch Heatran in Reversal Mountain. You can find it on Route 18 which you can only reach by entering the Hall of Fame.
Allows you to Battle and Catch Cresselia on the Marvelous Bridge. You can get this by going to the Strange House in the area outside of Reversal Mountain. You can only access Marvelous Bridge after you enter the Hall of Fame.
Allows you to merge Kyurem with Reshiram or Zekrom. You can only get this by Catching/Defeating Kyurem in the Giant Chasm for the second time. It drops it after the battle.
Have a Shaymin in your party and talk to the girl nearest to Nurse Joy in the Striaton City Pokémon Center. You can only reach Striaton City after entering the Hall of Fame.
Allows you to go to the Nature Preserve by Skyla's Plane. The Nature Preserve has a Level 60 Shiny Haxorus and contains many rare Pokémon like Kecleon. You can get it from Professor Juniper in Nuvema Town by showing her your Pokédex when you have seen all of the Pokémon in the Unova Dex, not counting Meloetta, Keldeo, Genesect, and Victini.
Makes eggs hatch faster. Show Professor Juniper, a complete Unova Dex, All Pokémon are caught, in Nuvema Town.
Makes Shiny Pokémon more common. Show Professor Juniper, not Cedric, a Complete National Pokédex in Nuvema Town..
Go to Abundant Shrine with Therian Landorus, and a n old man will give you this item, when you get to the wooden shrine. Use this to change Formes of Landorus, Thundurus and Tornadus.