Where gods have Power. Underdark Weave.

[prettystarprincess]

My suggestions.

Clerics of "Under dark" gods cannot cast 7-9 circle spells and epic spells on the surface.
Clerics of "Surface" gods likewise cannot cast 7-9 circle spells and epic spells in the Under dark.

Favored Souls cannot cast healing and restorative spells in their opposing environment. (healing, neutralize poison ect)

No Divine Might or Divine Shield in opposing environment.

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Mages have 5% spell failure in their opposing environment.

Under dark Races and Outcast are under dark mages. Everyone else is Surface mage.

You could go futher with this but really it's just to make a point of who has power where not make it impossible for people to travel to the other region. Also, rare items that would provide a necessary link to their god while on the other side would be a nice way to allow adverse traveling. Likewise mages could have to get some item too, those showed up in the books vaguely if I recall.

- My reasons

The gods who have power where seems be a a "lol" topic IC with so many surfacers in the Underdark constantly who you serve should effect your divine weave.

Faerzress was a magical radiation only found in the Underdark. It was said to be a remnant of the forces that shaped the Underdark terrain and it interfered with the casting of some spells... Underdarkers are accustomed to it and surfacers aren't. With that said Underdarks would have to ajust to casting withouut it.