French for devilry. In Vampire: The Masquerade there's a thing called Diablerie (among younger kindred: The Forbidden Slurp, No-No Nibble, reading from the OmNOMNOMnicon, The Spicy Succ) , which is essentially vampire cannibalism. Vampires can feed on each other just as well as they can mortals and animals, but draining another vampire to death allows the predator to consume the victim's soul and power as well as his vitae. If the victim is of an elder generation, then the predator can absorb that power, growing closer to Caine in the process (mechanically decreasing his Generation attribute, since being closer to one is better). This is, no bones about it, a horrific crime. It's pretty much always a Humanity violation, since, well, eating someone's soul is pretty awful if you're even a bit religious, and fucks you up even if you're not.
Because of the ramifications of such a shortcut to power, but also because literally any kind of upward social mobility is utter anathema to their cozy little pyramid scheme, the elders of the Camarilla rank diablerie as one of the most heinous crimes in Kindred society. Even though Tremere gained his Antediluvian-equivalent power by diablerizing Saulot, he was able to paint Saulot as a diabolist, trafficking with the powers of Hell, who needed to be destroyed and managed to wrangle himself out of annihilation.
Since anyone who commits diablerie has his crime readily visible in his aura, it is nearly impossible (barring a particular Merit) to hide the evidence of the act and having a black-streaked aura is sufficient evidence for a Prince to command a blood hunt against the diablerist. The proscriptions against diablerie mean that Princes tend to honor a writ of destruction called against a diablerist in another jurisdiction.
Being composed of vampiric anarchists, iconoclasts, and psychopaths, the Sabbat views diablerie as just another path to power. The early Sabbat destroyed the Lasombra Antediluvian through diablerie in 1413 and credited Gratiano de Veronese with the kill, though nobody can produce evidence of who actually consumed his/its soul; even if a group feeds communally, only one gets the juicy soul goodness of diablerie. The Assamite clan used to perform diablerie so often that they have the black aura permanently on them, which all other clans tend to view as a source for suspicion. This was the result of a curse that gave them all an insatiable thirst for and addiction to vampire blood, and stopped only when another curse made them all allergic. Sometimes diablerie goes horribly wrong, and the consumer becomes the consumed. This has definitely not happened to just about everyone who ever tried to diablerize an Antediluvian and don't worry about it. The other option is that the diablerist becomes per-verbatim possessed by their victim, essentially just becoming them. The higher in power the victim was, the greater chance of this phenomenon. When all is said, diablerie is a blunt and clumsy way to power with risks greater than the reward, like driving against traffic while speeding just to "get home faster".
Due to the changes in mechanics for Vampire: The Requiem, diablerie is a little different. The punishment for doing so remained unchanged, and the rewards aren't quite as appealing (essentially free XP and the ability to get Disciplines your Clan wouldn't be able to access normally vs. a power boost that was almost impossible to get any other way), and the odds of getting caught have greatly increased. That said, it's still a fast and effective way for an ambitious vampire to strengthen the Blood and there aren't many other ways to learn out-of-Clan Disciplines.
In Vampire the Masquerade: Redemption, you can diablerize an ancient Setite, losing just 1 point of Humanity and get a fuckton of powers, blood pool, experience and a whole Generation raise. You are a fucking moron if you don't do it.
In Vampire the Masquerade: Bloodlines, an INCREDIBLY good vidya gaem adaptation of VtM, the plot is, without spoiling anything, heavily centred on one or more characters attempting to perform diablerie on a supposed Antediluvian, hidden in a locked sarcophagus. Would it work? Who's trying to do this? Is there even an Antediluvian in the sarcophagus? You don't fucking know (atleast until the very end).