[Galadriel55]

Person - no
Place - no
Is it alive - no
Object - no
Law/ban - no
Natural - good question. No.
Doom/curse - no
Girdle of Melian - no, but its the closest answer so far.
Enacted by a Vala - possibly. I doubt that they were the first ones to ever "use" this.
Oath - no, but getting closer
Doors of Night - no, but also in the right direction.
Oath of Feanor - I said it's not an oath.

Time for another hint, perhaps. There are different kinds of concepts. There are abstract-concepts, like love and belief, that you can't really illustrate or imagine in a physical form; you just understand them (or don't). There are also those that, well, are more physical, like Girdle of Melian and Doors of Night. Although you can't really see/feel/etc them, they have a definite place, or a substence (not in these examples, though), or something else sort-of-physical in them. The thing I'm thinking of is the second kind of concept.
Also, the approach is more correct with the Girdle of Melian and The Doors of Night, but "oath" is closer in the sense of what it consists of...

It is so hard to give clues that aren't going to confuse others but won't give away the answer! Please tell me if you need clarification of this clue, and I'll try my best to give a better explanation.