To most vampires outside of the Nosferatu, they are believed to be merely a paranoid delusion of the hideous clan.
According to Nosferatu lore, the Founder of their clan gave birth to two major bloodlines: his firstborn children, the Nictuku, who are all Blood bound to him; and a second bloodline, descended from a tribal priestess (possibly Baba Yaga) who escaped his will and gave origin to the entirety of the modern Nosferatu clan.
The Founder did not take this disobedience lightly and considers that the modern clan is worth nothing more than extinction. The agents of this extinction are supposedly his firstborn children, the Nictuku, hideous creatures perpetually engaged in a crusade to wipe out the clan's strays. Some claim that the Antediluvian hopes that Caine will remove the curse upon him and that he will then be as beautiful as he once was.
In the Dark Ages, the Nictuku are a mysterious and deadly threat generally known only to some of the Nosferatu, though a few Cainite scholars (and those lucky enough to overhear the Priors' hushed whispers and rumours) prove the exception to the rule.
The Nictuku remain few in number but continue to hunt and kill any lone Nosferatu that they find, driving the Lepers deeper underground and into their communal havens. And though few Cainites realise it, the Nictuku do not hesitate to engage in diablerie with non-Nosferatu.
The Nosferatu treat the Nictuku seriously in the Final Nights; part of their fondness for sewers is because they build these underground havens into massive fortresses, complete with gun emplacements and embankments on the lower levels. The Nosferatu information exchange tools, up to and including ShreckNET, are intended to exchange Nictuku sightings.
For the most part, the Nictukus remain asleep. But according to the Masquerade of the Red Death Trilogy Vampire cosmogony, their awakening can be interpreted as the first step of the Gehenna. According to the Red Death Trilogy, Azazel is destroyed by a Mage, and Gorgo is buried under the Opera of Paris in the previous lair of the Nosferatu Fantomas. Also, Clanbook: Nosferatu Revised states Baba Yaga was killed. The whole event of her passing is described in the book Nights of Prophecy, where an ancient Nictuku named Vasilisa is revealed.
The word "Nictuku" supposedly came from a primeval word "nek-duku", meaning "they who guide death". Their entire system of belief centers around a supposed crime that the Nosferatu's progenitor committed; it was so terrible, so heinous, that every last one of the filthy Lepers should be destroyed for it. And yet despite the horror of that crime, its details has been largely forgotten by the bloodline that seeks to extract retribution for it. But the Hungry do not care; they will do whatever is necessary to make things "right", starting by slaying and devouring their kin, the Nosferatu.
The Hungry will not haven where they cannot see the moon when they rise and leave each night. They tend toward isolated locations in wild places near enough to the kine to feed and capture would-be-fledglings, but in places naturally dangerous enough to avoid most contact. Sheer cliff faces work well for the Hungry. They loathe the underground and will not haven there. Ever. This may be related to, or the cause of, the Nosferatu desire to dig.
Their prey is vampires, pure and simple. The Hungry get so little sustenance from the blood of kine that they must feed on vampires. Of course, they tend to be stationary and rarely venture from their territories in isolation. So where does the blood come from? Nosferatu whisper of kidnappings and Cainites of other clans held in perpetual captivity, fed by human cults who worship the Hungry. Similarly, they believe that the youngest assassins hunt in cities, bringing back the blood in their bodies for their sires.
They hunt, they kill, they eat other vampires, but they are at their most dedicated and cruel when on the hunt for a Prior. Because of their feeding habits, most members of the bloodline are diablerists, a state they seem to embrace rather than suffer from. They are all, by design or because of the Amaranth, of relatively lower generation, even the youngest.
For reasons best known to the bloodline alone, they tend to select the most beautiful youths they can find, grabbing up particularly fetching youths and maids from any village close to their havens. While healthy and capable specimens are ideal, sires favor beauty above all else, with a focus on youth and physical appearances far above the norm. They prefer these youths just at the cusp of adulthood, the blurry line between child and adult. The Hungry possess a great eeriness, though few see them long enough to put a finger on why. They are wide-eyed, their skin too perfect, their teeth too white, their fingers too long and graceful. Both their hunger and their beauty are inhuman.
Beyond their tendencies for whom to Embrace, little is known about what happens at the Embrace, though surely they must be starving for vampire blood at the moment they have turned. That hunger could lead to violence, and so it is likely that all Hungry undergo the blood oath before the Embrace or as part of the process. This ties into the Nosferatu theory that they build small cults in their isolated territories. Being bound at Embrace would also explain the bloodline's single-minded obsession.
Just before and after the Embrace, rigorous training starts to prepare the young assassins in how to hunt down and kill Nosferatu hold-outs.
The Hungry suffer from an inability to gain sustenance from kine blood. If a Nictuku were to drain a human completely, they would only be capable of drawing sustenance from roughly a third of the blood drained. However, Cainite vitae nourishes them normally.
Additionally, age is difficult and transforming on the bloodline. For every hundred years since their Embrace, the Hungry become more monstrous in appearance. But their physical bodies also grow stronger, until they reach the peak of human (and vampiric) physical potential. They grow slowly but decidedly bent, twisted, and ever more deadly, slowly aging grotesques that blame the Nosferatu for their condition. (Every 100 years take one dot from the Appearance Attribute, and move it to any Physical Attribute.) 
It is difficult to say if and how the bloodline organizes. There are blessedly few of them and their only interaction with Cainites is to feed or kill. It is possible that they form small cults of humans and young vampires around a central elder, but this is merely supposition.
According to the traditional Vampire: The Masquerade lore (from the 1st Edition, 2nd Edition, and Revised Edition), the Nictuku were all but myth until the book Nights of Prophecy presented a scenario where Vasilisa destroyed the Iron Hag. The Nictuku were believed to be composed exclusively of vampires of the fourth generation (a belief that was often disputed by the biography of Rasalon and his sire Androvikus, that appeared in the book Libellus Sanguinis 4: Thieves in the Night). Various Elder Nosferatu claimed the title "Nictuku" for themselves, in order to scare their descendants and to keep under their thumb.
However, Vampire: The Dark Ages 20th Anniversary Edition changed all that by presenting more detailed information about the bloodline (which was renamed as "Niktuku" with a "K"). According to the V20 version, the Niktuku are somehow related to the witch Baba Yaga, and can only sate their thirst with the blood of one kind of victim: other Cainites. Their origins remain questionable at best, according to the book itself.
Lore of the Clans presents multiple solutions: One is that the Nictuku are Nosferatu that are lost in Wassail and stalk their former brethren solely because of their bestial Hunger. Others say that they are literal monsters of human myths that the Nosferatu Antediluvian has under his thrall. Another presents the possibility that the conflation of Absimiliard with Nosferatu was wrong, and that there have been two Nosferatu progenitors that share the same Curse, one that is the leader of the Nictuku, and one that had been the lover of Arikel, drawing on Toreador myths about the Nosferatu Curse in order to explain why there are apparently Nosferatu of the fourth generation (the same record mentions that it seems more likely that such elders have reached their Generation by diablerie and are lying to cover up their crimes).