From Bulbapedia, the community-driven Pokémon encyclopedia.
Pledge Grove (Japanese: 誓いの林 Oath Woods) is a small forest-like location in southwest Unova. It is connected to Floccesy Town to the south. The path here is located directly to the right of Alder's house. The player's mom claims that the marks on the rock were made by Pokémon.
The main feature of the Pledge Grove is a rock with three cuts across it. If the player examines the rock with an Ordinary Form Keldeo in their party, Keldeo will want to come out of its Poké Ball. If the player lets it out, it will excitedly examine the rock, and then is reminded of Secret Sword by the determination of Cobalion, Virizion, and Terrakion that has been carved into the rock. If the player allows it to learn the move, the spirits of Cobalion, Virizion, and Terrakion appear, then zoom away; Keldeo resolves to battle, and is transformed into its Resolute Form.
In the manga
In the Pokémon Adventures manga
Pledge Grove was first featured in The Shadow Triad, where the Swords of Justice were seen teaching Keldeo how to use its sword. After one failed attempt, Keldeo managed to use its sword and leave a cut mark on the big rock, accompanied by its three mentors' cut marks. Afterwards, the four Pokémon left to follow the Shadow Triad whom Cobalion, Virizion, and Terrakion had encountered shortly before.
Pledge Grove was seen again in Pokédex Lecture, where Blake took Whitley there to register Pokémon in her Pokédex as a way to get closer to her. During their visit, however, they were attacked by Keldeo, accidentally attracted there by Whitley. Blake managed to avoid Keldeo's attack, but Whitley was knocked unconscious.
In New Sword Player, Blake revealed that he had later returned to Pledge Grove, offering Keldeo his alliance in order to help it develop its techniques. Agreeing to this, Keldeo had let itself be caught by Blake.
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