Five examples.
1. Frodo against the Nazgul before Rivendell.
Straight forward simple prophecy. Easy to miss. Not conclusive.Tolkien wrote:“Go back.”
“To Mordor we will take you.”
“By Luthien and Elbereth the fair, you shall have neither the ring nor me.”
2. Gandalf against the Balrog.
Again, a simple statement. Unfair as the Balrog is not taking part in the dialogue. A statement of what is to come.Tolkien wrote:""You cannot pass," he said. The orcs stood still, and a dead silence fell. "I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass."
3. Gandalf against the Witch King at the gates of Mines Tirith. The first complex dual between two adepts, and likely the most messy example.
Note, Gandalf with his prophecy was not just looking to turn the Witch King at the gate. He included Sauron in his curse. He was out to win the war then and there. The counter curse, to come true, required the Nazgul to kill Gandalf then and there. He didn’t. He also declared it his hour. Earlier in the books, it was established that in Gondor, the first hour of the day began with sunrise. The cock crowing indicated that the Witch King’s self proclaimed hour had ended.Tolkien wrote:“You cannot enter here,’ said Gandalf and the huge shadow halted. ‘Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your Master. Go!’
The black Rider flung back his hood, and behold! He had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter.
‘Old fool!’ He said. ‘Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!’ And with that he lifted high his sword and flames ran down the blade.
Gandalf did not move. And in that very moment, away behind in some courtyard of the City, a cock crowed. Shrill and clear he crowed, recking nothing of wizards or war, welcoming only the morning that in the sky far above the shadows of death was coming with the dawn.
And as if in answer there came from far away another note. Horns, horns, horns. In dark Mindolluin’s sides they dimly echoed. Great horns of the North wildly blowing. Rohan had come at last.
4. Eowyn against the Witch King.
The Nazgul knew he had lost the exchange of prophecies.Tolkien wrote:‘Begone, foul dwimmerlaik, lord of carrion! Leave the dead in peace!’
A cold voice answered: ‘Come not between the Nazgul and his prey! Or he will not slay thee in thy turn. He will bear thee away to the houses of lamentation, beyond all darkness, where thy flesh shall be devoured, and thy shriveled mind be left naked to the Lidless Eye.’
A sword rang as it was drawn. ‘Do what you will; but I will hinder it, if I may.’
‘Hinder me? Thou fool. No living man may hinder me!’
Then Merry heard of all sounds in that hour the strangest. It seemed that Dernhelm laughed, and the clear voice was like the ring of steel. ‘But no living man am I! You look upon a woman. Eowyn I am, Eomund’s daughter. You stand between me and my lord and kin. Begone, if you be not deathless! For living or dark undead, I will smite you, if you touch him.’
The winged creature screamed at her, but the Ringwraith made no answer, and was silent, as if in sudden doubt.
5. The One Ring against Gollum
After Gollum’s next to last fight against Frodo, the One Ring itself took a turn.
And there you have it. In all of fantasy, the dumbest possible prophecy.Tolkien wrote:Then suddenly, as before under the eves of the Emyn Muil, Sam saw these two rivals with other vision. A crouching shape, scarcely more than the shadow of a living thing, a creature now wholly ruined and defeated, yet fill with a hideous lust and rage; and before it stood stern, untouchable now by pity, a figure robed in white, but at its breast it held a wheel of fire. Out of the fire there spoke a commanding voice.
‘Begone, and trouble me no more! If you touch me ever again, you shall be cast yourself into the Fire of Doom.’