I know I need a hook in order to get the crystal from behind the grate, but where do I get this?

There's a locked chest in the room across from Rethpian's hangout. The stick inside will attract the crystal.

Xignal

welcome to the W&W forum! first of all, you need a bauble stick
you find a bauble stick in one of the chests


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now are you sure?
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It's in a chest in a room near Rethpian.