Directly outside the town exit of Freezington nearest to Peony's base camp cabin, you will find a small tree with Pokemon Footprints under it. As you approach this tree, Professor Sonia will suddenly appear. After getting reacquainted, she will tell of her theory about three Legendary Pokemon from the Unova Region having a secondary home on the Crown Tundra. She will ask that you search the Crown Tundra for the footprints of these three Pokemon - the Iron Will Pokemon, the Cavern Pokemon, and the Grassland Pokemon - asking for you to return to her in Freezington after you have found all of the footprints for each. There are a total of fifty footprints for each of these three Legendary Pokemon in question.
The Iron Will Pokemon is none other than Cobalion, one of three Pokemon apart of the Legendary Swords of Justice Trio from the Unova Region. In order to find all fifty of the Cobalion Footprints, you will need to extensively search the Frigid Sea, Roaring-Sea Caves and their given nearby areas. After finding all fifty of Cobalion's Footprints, return to Sonia in Freezington to update her. With this new bit of information, Sonia will tell you that you should be able to find Cobalion in the Frigid Sea Area.
The Cavern Pokemon is none other than Terrakion, one of three Pokemon apart of the Legendary Swords of Justice Trio from the Unova Region. In order to find all fifty of the Terrakion Footprints, you will need to extensively search Lakeside Cave, Ballimere Lake and their given nearby areas. After finding all fifty of Terrakion's Footprints, return to Sonia in Freezington to update her. With this new bit of information, Sonia will tell you that you should be able to find Terrakion in Lakeside Cave.
The Grassland Pokemon is none other than Virizion, one of three Pokemon apart of the Legendary Swords of Justice Trio from the Unova Region. In order to find all fifty of the Virizion Footprints, you will need to extensively search the Giant's Bed and its given nearby areas. Luckily, most of them should continue down certain paths, making it not all that difficult to find all fifty of them. With this new bit of information, Sonia will tell you that you should be able to find Virizion in the Giant's Bed Area.
Once you have captured these three Legendary Pokemon - Cobalion, Terrakion, and Virizion - head to Ballimere Lake. If you make a batch of Curry in the pot on Ballimere Lake while having the three Legendary Swords of Justice Trio in your party, Keldeo will appear. This is the first time that Keldeo, a Mythical Pokemon only previously obtained via Wi-Fi Events, can be found within the Overworld of a Mainstream Pokemon Game