The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.
At that moment Frodo threw himself forward on the ground, and he heard himself crying aloud: O Elbereth! Gilthoniel! At the same time he struck at the feet of his enemy. A shrill cry rang out in the night; and he felt a pain like a dart of poisoned ice pierce his left shoulder. Even as he swooned he caught, as through a swirling mist, a glimpse of Strider leaping out of the darkness with a flaming brand of wood in either hand. With a last effort Frodo, dropping his sword, slipped the Ring from his finger and closed his right hand tight upon it.

(LoTR, I-11)

Frodo dozed, though the pain of his wound was slowly growing, and a deadly chill was spreading from his shoulder to his arm and side.
...
'And more deadly to Frodo was this!' He stooped again and lifted up a long thin knife. There was a cold gleam in it. As Strider raised it they saw that near the end its edge was notched and the point was broken off. But even as he held it up in the growing light, they gazed in astonishment, for the blade seemed to melt, and vanished like a smoke in the air, leaving only the hilt in Strider's hand. 'Alas!' he cried. 'It was this accursed knife that gave the wound. Few now have the skill in healing to match such evil weapons.
...
The herb had also some power over the wound, for Frodo felt the pain and also the sense of frozen cold lessen in his side; but the life did not return to his arm, and he could not raise or use his hand.

Frodo was restless. The cold and wet had made his wound more painful than ever, and the ache and sense of deadly chill took away all sleep. He lay tossing and turning and listening fearfully to the stealthy night-noises: wind in chinks of rock, water dripping, a crack, the sudden rattling fall of a loosened stone. He felt that black shapes were advancing to smother him; but when he sat up he saw nothing but the back of Strider sitting hunched up, smoking his pipe, and watching. He lay down again and passed into an uneasy dream, in which he walked on the grass in his garden in the Shire, but it seemed faint and dim, less clear than the tall black shadows that stood looking over the hedge.
...
'What is the matter with my master?' asked Sam in a low voice, looking appealingly at Strider. 'His wound was small, and it is already closed. There's nothing to be seen but a cold white mark on his shoulder.'
'Frodo has been touched by the weapons of the Enemy,' said Strider, 'and there is some poison or evil at work that is beyond my skill to drive out. But do not give up hope, Sam!'

Briefly Strider told of the attack on their camp under Weathertop, and of the deadly knife. He drew out the hilt, which he had kept, and handed it to the Elf. Glorfindel shuddered as he took it, but he looked intently at it.
'There are evil things written on this hilt,' he said; 'though maybe your eyes cannot see them. Keep it, Aragorn, till we reach the house of Elrond! But be wary, and handle it as little as you may! Alas! the wounds of this weapon are beyond my skill to heal.

Frodo's pain had redoubled, and during the day things about him faded to shadows of ghostly grey. He almost welcomed the coming of night, for then the world seemed less pale and empty.

He could see them clearly now: they appeared to have cast aside their hoods and black cloaks, and they were robed in white and grey. Swords were naked in their pale hands; helms were on their heads. Their cold eyes glittered, and they called to him with fell voices.
...
With his last failing senses Frodo heard cries, and it seemed to him that he saw, beyond the Riders that hesitated on the shore, a shining figure of white light; and behind it ran small shadowy forms waving flames, that flared red in the grey mist that was falling over the world.

(LoTR, I-12)

`What happened at the Ford?' said Frodo. `It all seemed so dim somehow; and it still does.'
'Yes, it would. You were beginning to fade,' answered Gandalf. 'The wound was overcoming you at last. A few more hours and you would have been beyond our aid. But you have some strength in you, my dear hobbit!
...
Elrond is a master of healing, but the weapons of our Enemy are deadly. To tell you the truth, I had very little hope; for I suspected that there was some fragment of the blade still in the closed wound. But it could not be found until last night. Then Elrond removed a splinter. It was deeply buried. and it was working inwards.'
Frodo shuddered, remembering the cruel knife with notched blade that had vanished in Strider's hands. `Don't be alarmed!' said Gandalf. `It is gone now. It has been melted. And it seems that Hobbits fade very reluctantly. I have known strong warriors of the Big People who would quickly have been overcome by that splinter, which you bore for seventeen days.'
`What would they have done to me?' asked Frodo. `What were the Riders trying to do?'
'They tried to pierce your heart with a Morgul-knife which remains in the wound. If they had succeeded, you would have become like they are, only weaker and under their command. You would have became a wraith under the dominion of the Dark Lord; and he would have tormented you for trying to keep his Ring, if any greater torment were possible than being robbed of it and seeing it on his hand.'

(LoTR, II-1)

gul - 'sorcery' in Dol Guldur, Minas Morgul. This word was derived from the same ancient stem ngol- that ppears in Noldor; cf. Quenya nólë 'long study, lore, knowledge'. But the Sindarin word was darkened in sense by its frequent use in the compound morgul 'black arts'.

mor - 'dark' in Mordor, Morgoth, Moria, Moriquendi, Mormegil, Morwen, etc.