The Nine kings were given nine rings by Sauron. These rings modified them into the creatures that they became, creatures of darkness and evil. Their form or bodies perished and they had to wear armor and cloaks in order to be visible to others.
For many years the nine kings used these rings, which gained them
great wealth, prestige and power. However, the effect of the rings
made their bodily forms fade over time until they had become wraiths
entirely, and served only Sauron.
The Nazgûl were untouchable to mortal men, unless attacked with
Wraith means ghost, spirit or something out of shadow, shadowy.
They are the Nazgûl, Ringwraiths, neither living nor dead.
Since they cannot die, for they are not among living I will say that they can be "perished" or "ended", and use these word in this answer.
The Nine fear light, fire and water. They fear two of the key earthly elements of this world. They fear light and reside in shadow and cannot show a true form in this world. Hence the armor and cloaks which they wear. If we say they are the creatures of the darkness, than that statement would be true; but if we say they fear water because they are creature of the darkness, than that would be false for they also fear fire (we can associate fire with both good and bad). In the end some of them were "ended" by fire.
At this point, we can also speculate that they also fear the elements earth and air. Let me further explain this:
- Fire is primarily hot and secondarily dry.
- Air is primarily wet and secondarily hot.
- Water is primarily cold and secondarily wet.
- Earth is primarily dry and secondarily cold.
In LOTR we see that they fear forms of hot, cold and wet - from the above we can deduce that they also fear the dry form. They can only be "perished" by fire, or enhanced weapons (any of the above), but we can speculate that they can be "perished" by any of the above in an enhanced form. In truth, they fear all the elements of this world for they are the creatures of the shadow (the Wraith).
Furthermore there is another theory in which each of the elements have three properties, so we can provide a connection to the LOTRs "perishing" of the nine. Fire is sharp, subtle, and mobile while its opposite, earth, is blunt, dense, and immobile.
- Fire Sharp Subtle Mobile
- Air Blunt Subtle Mobile
- Water Blunt Dense Mobile
- Earth Blunt Dense Immobile
They can be "perished" or "ended" by fire. Which is Sharp, Subtle, Mobile, we can also find one of fire's properties in water Mobile.
So, in theory and as a connection to LOTR. Three of these properites need to be present in order to "perished" or "ended" a wright. We could see that in LOTR, but one or two of these properties can potentially harm it. Two of them seen in air and one of them in water.
From this we can deduce that they cannot be "perished" or "ended" by water but they can be harmed by it.
They seemed to be unwilling to cross running water, although they
could if they had to. Their greatest weakness was apparently fire.
Additionally, the Nine cannot see like men do and don't like to walk and hunt in daylight, but at night in shadow.
The Nine could communicate telepathically. They do not see during the
day as mortals do; instead they see shadowy forms. During the night they see many signs and forms invisible to mortal eyes; it is at night that they are to be feared most.
Crossing the river for them was a nuisance.
"The Nine are abroad again. They have crossed the River secretly and
are moving westward. They have taken the guise of riders in black."
—Radagast to Gandalf.
Provoked, the Nazgûl crossed the river to take the Ring by force from
a weak and injured Frodo. However, the water, enchanted by Elrond and
Gandalf, formed a great wave and swept the Nine away, killing their
They crossed the river in fear, anger and disgust, but they knew that it wouldn't kill them in the end. It would mean the end to their horses (enhanced by evil, but still horses of this world) and their hunt, but not them. They knew they could lose this battle if they crossed it, but it was all that they could do at that point in time, for they are drawn to the One Ring like nothing else in this and their world (the world of shadow). In truth, they had to cross the river, they had no choice, for Sauron, and for the One Ring.
the Nazgûl were forced to retreat to Mordor on foot and stop their
hunt for the Ring
After this, the Nine received improved new mounts that wouldn't stop them at rivers, the Fell beasts.
They were surrounded by an aura of terror, which affected all living
creatures; their aura (called the Black Breath) could be toxic to
those hapless enough to come near them.
This aura also protects them from the properties in elements, which I have talked about above in this answer. Their aura can poison water and pollute the air around them. Leaving them unharmed and protected.
They were physically weaker in the LOTR - Fellowship of the Ring, as also their aura was weaker. We could see that from the crossing of the water and small portions of fire when Aragorn drove them away with two burning sticks.
they would become vastly more powerful. However, it is unclear as to
how. However, as Sauron's strength grew through the books, the Nazgûl
became obviously more powerful. In the Fellowship of the Ring, the
Nazgûl's cries were simply unnerving to the hobbits (this may possibly
be explained because it was important that the Hunt of the Ring
remained in secrecy so they might have diminished their auras, and
they did not have the rings of power), and they appeared to be
physically weak, as five of them were driven off by Aragorn with two
burning sticks. Additionally, Gandalf the Grey managed to hold off the
entire Nine single handedly on Weathertop. In The Return of the King
however, their cries are powerful enough to send all but the most
stout-hearted of Gondor's defenders into a state of helpless terror,
and the Witch King in particular has become so powerful that he is a
match for Gandalf the White (which of the two is the more powerful is
Situation which you are referring to with the ferry, only happened in the movie and not the book. For in the book the Nazgul arrive later and find that there are no more boats left for them to cross the river on.
En route to the new house at Crickhollow, Frodo, Sam, Merry and Pippin
crossed using the Ferry just before the arrival of a Black Rider, who
was forced to go around to the Brandywine Bridge as there were no
boats kept on the western bank of the river. (In the film version by
Peter Jackson, the encounter is more immediate.)