I don't have the book handy, so whoever gets to this first can fill in the curse that Hathil's brother makes|
When Orban attempts to steal the Dorig prince's collar, the Dorig tries to dissuade him by weaving a curse on the collar.|
The curse will bring ill fortune to the possessor of the collar, all the time clouding his mind and causing it to cling to it as his most valuable possession.
The Dorig ties each part of the collar's decorative pattern into the curse, each representing a different kind of misfortune.
With his last words, the Dorig seals the curse by calling on the Old Power, the Middle, and the New, so that the curse can not be released until all three are placated. This makes it a powerful curse indeed - the more powerful because it was a dying curse as well.
Orban takes the cursed collar home, from where the curse spreads to affect all the Otmounders, then all the Lymen, and eventually all the people of the Moor, including the Giants and even the Dorig.
Orban's sister, Adara, swears that she will try and appease the Powers and break the curse.
The Dorig's brother, Hathil, swears that he will help the curse every way he can, and that he will spill every drop of Orban's blood (except Adara's); and calls on the Powers not to be satisfied until none of Orban's people are left alive on the Moor.
For what became of the collar and the curse in the end, see Power of Three (spoilers)