The closest equivalent in Pokémon Diamond would be the Summit, which is the final location in the game where you fight the Galactic Leader Cyrus and the legendary Pokémon Dialga or Palkia.
To get to the Summit in Pokémon Diamond:
1. Complete the main story: You must first complete the main storyline, including defeating the Elite Four and Champion.
2. Head to the top of Mount Coronet: Once you have completed the main storyline, you can access the Summit by traveling to the top of Mount Coronet.
3. Follow the path to the Summit: The Summit is located at the very top of Mount Coronet. You will need to follow the path upwards, battling various trainers and wild Pokemon along the way.
Let me know if you have any other questions about Pokémon Diamond!