I noticed that this still grants Call Lightning, but it doesn't really make sense anymore.
Call Lightning, in the UD, was altered to cause a cave in, instead. But since we've moved back up to the surface, this change was reverted.
Or, was Call Lightning changed to detect if the caster is indoors or out, and change the effect accordingly?
If not, I'd suggest it do just that. Or if that is too complicated, Cavern Domain ought to be given something more appropriate. Though, I'm stuck as to ideas in that regard...
Bombardment instead?
that'd be good, although it would have to be toned down a bit to be at a reasonable level for the server. Bombardment is a level 8 spell. Match it's damage to call lightning and it could be given to Cavern's Domain at the same level as Call Lightning
Stoneskin?
Actually, since the domain seems to be a lot about detection in and sneaking around in caves, how about forgetting the damage spell and going with Find Traps as a Level 1 spell?
http://nwn.wikia.com/wiki/Bombardment
Not that overpowered at caster level 7.
Yeah, maybe not Relinquish.
As for stoneskin, I'd have to say no. Earth, Strength, Magic, and Good all already get that.
However, maybe some kind of stealth bonus while in caves could be interesting. Or just Bombardment as a replacement. I didn't realize that in the UD call lightning was essentially replaced for bombardment, damage and all. I assumed it was just the graphic and damage type, but the amount stayed the same.
If it wasn't over powered there, it can't really be considered over powered here. Especially when you consider how few places it can be used in comparison to the UD or even call lightning.