The power of a curse

[Artanis]

In the Sil project we have been talking about the curse of Morgoth upon Húrin and his kin, and this has made me think about curses in general. What is the power behind a curse?

This is the curse of Morgoth:

Therefore Húrin was brought before Morgoth, for Morgoth knew that he had the friendship of the King of Gondolin; but Húrin defied him, and mocked him. Then Morgoth cursed Húrin and Morwen and their offspring, and set a doom upon them of darkness and sorrow; and taking Húrin from prison he set him in a chair of stone upon a high place of Thangorodrim. There he was bound by the power of Morgoth, and Morgoth standing beside him cursed him again; and he said: 'Sit now there; and look out upon the lands where evil and despair shall come upon those whom thou lovest. Thou hast dared to mock me, and to question the power of Melkor, Master of the fates of Arda. Therefore with my eyes thou shalt see, and with my ears thou shalt hear; and never shalt thou move from this place until all is fulfilled unto its bitter end.'

In the case of Morgoth and Húrin it seemed that Morgoth was powerful and cunning enough to make his curse come true by careful planning and manipulation of events. But then we have the curse of Isildur, who cursed the Men of the Mountains when they refused to go to war with him against Sauron, though they had swore allegiance to him.

‘Then Isildur said to their king: “Thou shalt be the last king. And if the West prove mightier than thy Black Master, this curse I lay upon thee and thy folk: to rest never until your oath is fulfilled. For this war will last through years uncounted, and you shall be summoned once again ere the end.”

As we know, the spirits of these men were doomed to remain until Aragorn summoned them to the battle of Pelargir. What power had Isildur to fulfil this curse, to put this doom upon those men?

There's another example in UT, where the Dwarf Mîm lays a curse upon the Man who killed his son:

But this I will add: he that loosed the shaft shall break his bow and his arrows and lay them at my son's feet; and he shall never take arrow nor bear bow again. If he does, he shall die by it. That curse I lay on him."

This curse was also fulfilled at last. A coincidence? Or did Mîm have some inherent power to make it come true?

Thoughts, anyone?