Over the years I've revised some GURPS Magic spells for my DF Felltower game.
The Sunlight spell is an especially weird spell, in my experience.
Part of this is because of how games treat darkness - darkness is more of a tangible thing, like smoke, just without the actual smoke. It looks black, you can't see through it, it can have a shape and limits. It's like squid ink in air, really, just not substantive. Light, though, we know diffuses out over and area, and it is brighter near the source and less out from it, and can be seen from far away.
Darkness, thus, works not at all like in reality, where it is an absence of light, but we expect light to work just like in reality. It creates a lot of confusing and conflicting expectations.
Add in, then, the sunlight spell. As written, it creates "real" sunlight, for purposes of anything that is affected by sunlight.
It also creates a spell that extends upwards as far as possible - the sky, to clouds above, to a ceiling if cast inside or underground. But does it extend down, if cast higher up? The spell doesn't say yes or no.
If yes, you get delvers casting the spell on a hole in the ground at -0 for range and getting a shaft of sunlight that extends up and down as far as it can. Got a nearly bottomless pit? Why drop down an object with Continual Light on it when you can spend 2 energy and cast Sunlight and shine a shaft of light all the way down to the bottom and harm any vampires intersected by the path as a bonus. Not sure where the roof is? Cast sunlight and it'll extend upwards, too. The latter is clearly not as big of an issue as the former can be. Especially when the former goes through a No Mana Zone - is it natural sunlight, and thus ignores it, or magical, and thus ends on it and reveals it?
If no, you get the oddness of a hard stop of sunlight on nothing. That violates the way light operates in the game otherwise, where it comes from a source and diffuses out.
There isn't an easy solution here.
- If I treat Sunlight like Darkness, it is just a small area of light with sharply defined boundaries but is totally inconsistent with everything else we do with light.
- If I allow it to extend up and down, but it's really annoying that it does. It becomes a really effective flashlight and people "explore" by shining a perfect, -0 to see light down to look around.
- I could substantially change the spell to create a sunlight source, much like Continual Light, but then people will cast it on rocks and carry sunlight around.
- I could allow it only to be cast at the terminus of light. So, no shining down by casting it up in the air to look down. This has the benefit of being as written even if not as intended. You simply couldn't cast it if you weren't on the ground or some other ground surface.
I'm not sure how I want to deal with this. I think the last one might be the only way to "easily" fix the spell without having the weirdness of effectively always being only horiontally-penalized for distance as it'll always been up and down.
I'll have to give it some thought, as it's been a minor but annoying issue for all of us in the DF Felltower campaign.