Here is a article on
respecting science. The site, named Atomic Rockets, is written for a science fiction audience, but I think that article has advice that is relevant to the current debate.
As a rule of thumb, the more fundamental the theory is that you just broke, the more serious and the more numerous will be the unintended consequences.
Back to undead. At present, they operate with no food or energy source, in direct violation of Conservation of Energy. So do amethyst men, magma men, and others. Conservation of Energy, also known as the First Law of Thermodynamics, is a very fundamental law of physics. As such, breaking it will have implications for all the physics statements and theories which assume it, and are built on it (EVERYTHING, or near enough as makes very little difference). This represents a long and diverse list of "unintended consequences," which I referred to earlier with the phrase "collateral weirdness." For this reason, I want to keep it fully intact and unmodified at all costs.
As I am describing it, "Magic fields" involve four distinct "miracles." These combine into a single mostly coherent system.
1. The magic field itself, which serves as both
wireless power transmission and energy storage. It probably represents a new field within physics.
2. An energy receiver, in the form of a biochemical pathway (xenosynthesis). In other words, a tweak within biochemistry.
3. An energy producer and transmitter, in the form of another biochemical pathway (call it "alchemical plants" for now). In other words, another tweak within biochemistry, closely related to the previous one.
4. A different receiver, exploited by non-biological "creatures" such as undead, amethyst men, and blizzard men. I'm not sure where this one lands.
Yes, magic energy does represent a significant departure from real-world physics: a method of power transmission and storage that does not follow the rules we know. However, this description permits it to be analyzed and described by experiment, so the Scientific Method remains fully intact and functional. "There are rules in here somewhere; we just need to figure them out."
Summary: I consider magic fields to be much less unrealistic than permitting a violation of Thermodynamics. The consequences are intended (the xenosynthetic cavern ecology, for example) or, failing that, relatively predictable.
Even if you can justify sun-less plants and undead by the same miracle (magic metabolism), it's no better of a solution than the other for total internal consistencies. And the lesser magic is less hand wavey.
I think magic fields ARE the "less hand wavey" solution to undead and the others. Sun-less plants are merely a very nice bonus.
Amethyst men can probably go, even if we want to keep undead, etc.
Why? Amethyst men use the same basic miracle as undead, both in my system above (number 4) and in the game as it currently stands.
there are already like 7 fairly unrelated miracles in game, reducing existing ones if possible without it being much less fun is ideal.
Can you give me a list? It would make reducing the number a lot easier.
On to other objections.
Sunlight is needed for those, but it's not difficult to imagine geothermal energy being the driver instead, especially since the biology of such is already proven on earth.
Do you have a source for the biology comment? Using heat as an energy source is not the same thing as surviving and operating at high temperatures.
I also dispute your "not difficult to imagine geothermal energy" claim. Heat by itself is not something that can be exploited as an energy source. It requires a HEAT GRADIENT: heat flowing from a hot "source" to a cold "sink." Bacteria are small, almost certainly too small to have simultaneous access to both. Larger "plants" would still need both a source and a sink, which would probably involve tunneling through rock. This feat is both difficult and slow. Even if it the plant managed to avoid that issue, it would probably still need to achieve substantial length (requiring a correspondingly substantial initial investment) to obtain a usable heat flow. What energy source is being used for these investments?
I don't see why grouping undead and evil plants as an evil cluster is more efficient or meaningful than leaving undead and grouping all plants as a cluster.
This was a reply to your comment about "no plants in the circus." Undead, unlike clowns, have plants in the vicinity. You were right about that cluster not being meaningful. Now, with magic fields elaborated in more detail, they are potential candidates for miracle 3.
The places where "suitable" water can enter the cavern system are fairly limited.
Fair enough. But that also keeps organic matter inside from leaving with the same efficiency.
Organic matter leaving isn't really the point. Organic matter is a usable energy source, but that energy will inevitably get consumed. The atoms need to get recycled into a high-energy molecule. You have managed to push chemosynthesis from "out of the question" to "dubious," and have made a suggestion about using heat that I am even more dubious about.
Breathable air does not imply "outdoors" or connection to outdoors. It only implies "balanced ecosystem."
As far as I know, chemosynthesis does not have a pathway that generates molecular oxygen. The "thermo-synthesis" you describe seems even more unlikely to do it: generating O2 requires splitting water and/or CO2, both very stable molecules. Photosynthesis does this by exploiting the particle nature of light; heat gradients don't offer that feature. The oxygen needs to come from somewhere.