Here's why:
* Cobalion is a Legendary Pokemon: You can only obtain it by completing a specific event in Pokemon Black. This event involves interacting with the other two Swords of Justice, Virizion and Terrakion, and completing a series of tasks.
* The Boulder is not involved: The Boulder is a key item in the "Pokemon Black and White" games, but it's not related to Cobalion or the Swords of Justice event. The Boulder is used to open a specific area and trigger a different event.
To get Cobalion in Pokemon Black, you need to:
1. Obtain the "Truth" event: This event is no longer available in the original games, as it was a limited-time event. However, you can potentially access it via methods like hacking or using emulators with modified ROMs.
2. Find Virizion and Terrakion: These two Legendary Pokemon are located in specific areas of the game world, and you'll need to find them before you can trigger the Cobalion event.
3. Complete the event tasks: These tasks involve completing battles and solving puzzles, and you'll need to successfully complete them to encounter Cobalion.
Please note: It's important to remember that using modified game files or accessing content outside of official distribution channels can be considered illegal or against the terms of service for Pokemon games.