From Path of the Vampire Wiki
Necuratism is the practice of vampire cannibalism. The word comes from the Romanian "necurat", meaning unclean, probably because ancient vampires saw the practice as one of the worst imaginable crimes for a vampire. One who engages in necuratism is called a Necuratist, or a Necurat. (slang: 'N-Rat'). Engaging in Necuratism is most generally considered a heinous act and is most often kept hush-hush among other vampires.
It is unknown who was the first Necuratist of ancient times; the practice seemed to develop over time in small groups of rogue covens. In modern times, Emanuel Broussard re-discovered the lost skill, and passed it onto some of his gang's leaders.
One does not become a Necuratist simply by walking up to any vampire and drinking their blood. A vampire's body will reject vampire blood, causing them to become ill as though poisoned, if drunk in large quantities. In fact, vampire blood acts like poison in many other ways, because in very small amounts the blood can cause sensations of pleasure, much like some poisons do in small doses, which is why companions often drink tiny amounts from one another. But, like poison, you need to build up a tolerance of it in order to absorb more than a few drops, which eventually changes your body so that it can only survive on vampire blood.
That is where the training of a would-be Necurat comes in. First, one must ingest small quantities of their own blood. It is thought that in doing so, the vampires supernatural body is changed slightly. Although it is difficult at first, and causes weak vampires to become quite ill, eventually the vampire will be able to ingest small amounts of other people's blood without it having any effect at all. After some time being weaned onto other vampire's blood, they become able to ingest full pints of vampire blood. This is when the full change occurs, and the vampire is then irrevocably restricted to vampire blood alone.
A Necuratist immediately becomes much harder to kill. Their blood will drain from their body more slowly, and serious wounds will not effect them as much as they do with other vampires. However, they gain no other immediate power boost, and must work at growing stronger or mastering their powers just the same as other vampires do.
Necuratists can only drink blood from other vampires. This causes problems for them because in order to survive they need to find regular, willing donors. As a result, many vampires mistrust or attack Necuratists because they are in essence placing themselves higher on the food chain than vampires.
As always with these kinds of details, it is up to you to decide if your Necuratist can control their hunger, or if they go insane, "imagining" voices of the dead, etc. This is only a roleplay guide of beliefs held by some vampires.
This rumour is mostly true (though not ALL details), but we won't reveal the full reasons yet. Also, the Necuratist insanity only happens in about 50% of cases; usually only those who feed in excess or to the point that a vampire is drained completely.
Many vampires believe that Necuratists often go insane, either due to their enhanced link to the shadow realm, or because of their insatiable hunger. While there are documented cases of insane Necuratists, it is also true that most vampires of certain paths seem to go insane eventually anyway.
One myth holds that, when they ingest the blood of their own kind and their body is changed, they become abominations, linked to the realm of the dead (AKA the shadow realm) and all the wraiths who reside there. Those who believe that Necuratists are insane abominations also often believe that the shadow realm has such a grip on them that the voices of the dead whisper to the cannibals constantly, either consciously or subconsciously, causing them to either go insane or to succumb to the desires of the wraiths that speak to them, who are said to compel them to feed or sire as much as possible. This link is also seen to be the reason that modern Necuratists cannot return from the dead.
While the myth might seem to be superstition at best, there seems to be something substantial behind it, and some examples from history who apparently back up the belief. It is a known fact (proven by modern vampries) that the proximity of the shadow realm sundering grants vampires greater power, and allows them to grow more quickly. The tare in the veil between realms has granted vampires of Harper Rock with unprecedented growth speed, which would have usually taken the ancient vampires hundreds of years to attain. By that logic, then, it's easy to understand why some vampires could see the connection between the increased growth speed of a Necuratist, and the shadow realm's power over living vampires.
This myth is exaggerated, but true. Necuratists hunger far more than normal vampires, and can only control their addiction with serious discipline. Some do go mad with hunger, but most simply crave vampire blood to the point where it dominates their thinking most of the time.
Necuratists are said to have an increased hunger when they turn to vampire blood, even moreso than the considerable hunger found in vampires. Not all Necuratists seem able to deal with this very powerful and constant urge, and some have even been known to go entirely feral, putting the lives of all vampires in danger.
The rumour of taint is VERY exaggerated due to ancient vampire religions twisting the facts. While they are tainted in that they aren't normal vampires anymore, the rumours of them attracting fae and wraiths is untrue, though there seems to be some truth in the fact that Fade Beasts are more drawn to them. There is also no truth in the myth that zombies and spirits rise wherever they go. This myth was started when a certain Necuratist raised an army of zombies in the middle ages.
Some ancient vampires before the holocaust dogmatically guided the popular view of Necuratists into demonized creatures of evil. While this may or may not be true, the rumours of them attracting fae, wraiths and fadebeasts with their "tainted" presence seemed to get more and more elaborate with each telling.
There are also myths about how Necuratists unwittingly raise hordes of zombies and masses of ghosts wherever the go, as if they were some kind of magnets for the supernatural, endangering the secrecy of vampirism and the lives of other vampires.