Why can’t I re visit the volatiles nest to grab the shadow stone for the Excalibur?

[tardy bay]

I’ve done the main mission but didn’t realized the stone was there until now I kinda regret not knowing before Soo is there a way to go back?

[empty prism]

The shadow stone isn't even in one of the caves

[grand coyote]

It's not in the Volatiles nest. It's in ||The Exiles Cave||

[empty prism]

But you can always go in there