Personally, I think a curse is like an ultimate oath, or maybe an oath with a definate consequence. In ME, as well as most classical mythological stories and the Bible, if you break an oath some type of retribution will result. A curse, at least in Isildur's case, is what happens after an oath is broken. The point of Isildur's curse is to not give you're word lightly or dishonestly or Aragorn will come a few 1,000 years later and MAKE you fulfill your oath. That's my version of it anyways,
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