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: "Rat"). They gain power much faster than other vampires, and are much harder to kill.
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. The only difference is that they will discover new abilities at roughly twice the speed of other vampires, and will even grow in strength and intelligence at a slightly faster rate.
When they die, they are dead. Even with the sundering, and the rip in the veil between the real world and the shadow realm, they still cannot return to life as other vampires can. It is thought that the reason for this is that their tie to the shadow realm becomes so potent that it holds a greater grip on them (see "possible curses" for more details on this).
Additionally, a Necuratist 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.