If my character has committed diablerie in his past, does the taint in his aura, visible to auspex users, last forever? Can it be cleansed?
From page 159 of the Vampire: the Requiem core rules:
Finally, the act of Amaranth stains the diablerist’s own soul upon its commission. The character’s aura acquires black veins that reveal her crime to those who can scrutinize such things (see p. 120). These black veins remain in the diablerist’s aura for one year per dot of Blood Potency the victim possessed. This time is cumulative; a vampire who diablerizes two Kindred of Blood Potency 6 has veins in his aura for 12 years. If a character diablerizes his victims years apart, the additional years add on to the end of last. For example, if a Kindred diablerizes a vampire of Blood Potency 4 and two year later diablerizes a vampire of Blood Potency 5, the black veins appear in her aura for seven years following the second diablerie — or longer, if she keeps to her wicked ways and diablerizes again.
The taint will fade in a number of years depending on the victims blood potency.
From the VTR wiki, highlighted for relevance.
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.
I don't have the VTR book to hand to quote a page reference, but this is in line with what I remember.
What is diablerie?
It's defined in the rules as "draining another vampire of blood and then devouring their soul". This is why you have a mostly accepted house rule that the soul or part of it, of the vampire eaten get assimilated by the eater. If that is true, then I would say that the aura vanishes over time as the last of the soul fragments are either assimilated or take over the eater's soul over. How long that takes is probably dependent on the relative generations, age of both participants, willpower, and a whole lot of story driven factors.
What if the diablerie explanation was a lie? To whom does the standard definition benefit? The young vampire become a better, brighter, and more responsible vampire? Or the elder that does not want to get eaten? What if, there is no such thing as eating the soul, just taking their power? And if that's true, surely the aura would dissipate when the eater has accepted the power and mastered it. Besides, the "aura" might just be the guilt of doing something you were told was bad.
In all my vampire games, I kept this duality alive and well as I thought it added to the whole dark themes and ambiguous morals. While clearly somewhat anathema to the RAW, I believe adding the uncertainty makes for more interesting story lines.