One issue that I find a little troubling is that creation of permenant items seems to provide something for nothing when scaled out over time.
As far as I am concerned, the Conservation of Energy law needs to apply to magic as well as normal physics, though with greater flexibility - ‘energy’ can be drawn from more sources then conventional physics allows. Perhaps a better term would be ‘cost’ instead of ‘energy’.
A magic sword, when created, seems to have limitless power without any indication as to where this awesome power is drawn from.
Now, when an item is magical or subjected to magic effects, it can either be a once-ever transformation, or a sustained magical effect, or a combination of both.
A sword magically enchanted to have an atom-thin edge with ridiculous sharpness would be nice, but the lifespan of the wonderful edge would be very limited without ongoing enchantments to protect it - the wonderful edge would be rather delicate and not survive use.
For purposes of argument, if the end result is an item which can exist in a universe and follow conventional laws of physics, then a one-time enchantment is possible - a simple ‘transform’ - changing the properties, shape or quantity of matter. For example, one could make an item invisible simply by changing it’s property to not reflecting light and making it absolutely transparent. This would not require magic to sustain (Air is invisible, so a precendance exists), but this very literal effect would introduce other problems. For example, a person made invisible by transformation would be unable to see. ‘Smarter’ magic which does away with side effects such as this require ongoing expenditure of energy to sustain.
However, if the end result is an effect which continues to violate the laws of physics, then some magical means to sustain it must be provided.
For example a sword which provides a +5% magical enchantment, forever.
How does it achive this bonus? Is it extra sharp? Extra heavy? Does it lend mystical skill to the wielder? Does it seek ‘life’ and is drawn towards it in motion?
If the blade is simply magically sharpened, then it is really little more then a well-crafted sword with an unusual creation method, but if it is truely enchanted to add to the expertise of the wielder, then some source of this needs to be provided.
The power source of the sword obviously could be either internal or external. Internal could be a battery of magical energy which was infused into the sword, or perhaps a spirit of some type. In any case, an internally powered magical devise will eventually draw down it’s source.
If one says "What if the spirit is immortal?" then how is that immortality sustained? If it is of a divine source, then it is likely in truth externally sourced .
So, possible laws of magic:
1.Magic is the application of mystical force - Magum.
2.Magum is tied to Life and is present in any world where it exists.
3.Constructs or conditions not allowed by the Laws of Nature require Magum to create and sustain.
4.The greater the divergence from the Laws of Nature, the greater the cost in Magum required to produce and sustain the change.
If the end result is creating something which can exist according to the laws of nature, then the effect does not need
All things have cost
Nothing is eternal - even the Gods may pass on.
Additional Ideas (3)
Gods do not have a monopoly on the energy generated by the faith of sentiant beings. Many 'Holy' or unholy relics come into being not from the direct intervention of the divine, but through the sincere belief of the masses. If enough followers can be convinced of the power of an item, then this faith will help make this a self-fufilling prophesy. Obviously this type of energy source is generally beyond the reach of normal PCs', but kings, high preists, and even demons could make use of this to create and power items.
One might be by the faith of sentient being in their god, but if the religion dies so does the item, unless it is reborn... also some magical items where not permanent but the power of the legend around gave the item permanent power as long as the story was believed and retold (everyone around here knows about the Sword of Truth) taking power from belief and shaping it through the magic item.
Another is taking the power from lay lines, ambient magic, ambient life or pulling it from another realm altogether. This requires some form of collection device which may or may not be a part of the magic item itself. A sword that kills grass and bugs, or take the death of a small animal to work. Or a staff that is tied to an alter, destroy the alter and the staff is useless. (These are my favorite methods)
Something else that might do it is to imprison the soul of sentient being in the item to give its power. That should be permanent.
1 take bar of steel
2 magnetise using electric coils/other magnet (you get the idea)
3 permanent magnet
1 take normal sword
2 enchant using acrane ritual
3 sword with dangerous, life taking field around it.
First example is a verifiable physical process which produces a permanent effect, second one is a magical process which produces a permanent effect.
We could also consider the halflives of various radioactive compounds. 28000 years is a long time for a sword to become half as effective as when it was originally created in the magical forge.
So an enchantment could be a sort of irradiation process or magnetisation process. In one case, the magical effects would gradually weaken, but for human timescales, they would seem eternal. Magic could affect an until-now unheard-of fundamental particle, which would decay and release magical energy.