How do you i unlock the cave passage on way to spear pillar Pokemon diamond?

You don't unlock the cave passage on the way to the Spear Pillar in Pokemon Diamond. There is no passage that requires unlocking on that route.

Here's the route to the Spear Pillar:

1. Enter the Valley Windworks: This is located at the northernmost point of the Valley Windworks.

2. Follow the path: The path leads you through a series of winding tunnels and caves.

3. Climb the stairs: Eventually, you will reach a set of stairs that will take you to the Spear Pillar.

Note: There are no puzzles or special items required to progress to the Spear Pillar. You simply need to follow the path and climb the stairs.