Diablerie is the heinous act of consuming a fellow vampire completely, down to the victim's very soul. The act of diablerie can increase a character's Blood Potency and grant them access to powers and Disciplines known to the victim, but at a terrible cost to the character's Humanity. Diablerie leaves characteristic black veins on the aura of a Kindred observed through the Auspex Discipline. A Kindred who commits diablerie is called as diablerist. (VTR: Vampire: The Requiem Rulebook 
, p 158)
Diablerists generally perform such an act for one of two reasons - to gain in power by raising their Blood Potency or because they are addicted. The former tends to be more careful when choosing victims, as diablerie must be performed on vampires in torpor; nothing is as attractive to power-hungry neonates as the rumors of an elder in torpor. However, it is not unknown for a diablerist to beat their victim into torpor themselves before performing the act.
While the benefits of performing diablerie are many, among them increased Blood Potency and the chance of gaining new skill in disciplines, the drawbacks are considerable - the diablerist's Humanity automatically sinks, and black veins dot their aura for a number of years equal to the victim's Blood Potency, making them walking targets for Auspex users. Most are careful not to be seen.
Diablerie is addictive the same way that vitae is addictive, especially to elders (who usually must drink vampire blood just to survive). Diablerie-addicted elders are feared by many vampires.
Diablerie is also known by the older term, Amaranth. Notably, Amaranth is the Third Kindred Tradition, after the Tradition of Masquerade and the Tradition of Progeny. The traditional wording of this commandment is: "You are forbidden from devouring the heartsblood of another of your kind. If you violate this commandment, the Beast calls to your own blood."
Amaranth is the old form of what the Kindred now call diablerie. It refers to an ancient practice of giving an amaranth (a flower) to the victim-to-be. (VTR: Vampire: The Requiem Rulebook , p 170) Moreover, the word amaranth comes from Ancient Greek and means "everlasting flower" or eternal, undying, unfading, unwilting...