Adding fog to specific tiles?

How would I go about adding some fog to specific tiles of a map? I want to add some kind of depth effect to show that this map is in a very high area with a crevice in the middle, but I don't want the middle part to be just black.

A fog will overlay the whole map. Use a panorama instead.

Franzo said:

A fog will overlay the whole map. Use a panorama instead.

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Thank you so much! This is exactly what I needed.