The Spear Pillar is a location in the game that you can access at a specific point in the story, after defeating the Elite Four and becoming the Champion.
Here's how to reach the Spear Pillar:
1. Defeat the Elite Four and become the Champion.
2. Talk to Cynthia in her room in the Champion's Chamber. She will mention that she needs to go to the Spear Pillar to meet with her grandmother, and will suggest you join her.
3. Head to the top of Mount Coronet, where you fought the Legendary Pokémon Dialga or Palkia.
4. The Spear Pillar will be accessible, appearing as a large, stone structure.
Once at the Spear Pillar, you'll encounter a new challenge and face off against Cynthia again, this time with a different team and with a new Legendary Pokémon.
Remember: You can't open the Spear Pillar, you simply need to progress through the main story to reach it.