What are the cases of European nobility and aristocracy participating in satanism and the occult, where they kill or torture peasantry and servants - similar to Elizabeth Bathory?

You may not care very much, if your interest is purely in writing fiction, but historians urge significant care in assessing the famous cases of Gilles de Rais and Elizabeth Bathory, both of whom have been accused of sorcery, heresy, and hence involvement in "the occult."

There are several reasons for this. First, and probably most importantly, both de Rais and Bathory were important and wealthy landowners, and the addition of charges of sorcery, heresy and so on to those of murder made it significantly easier for some the powerful figures involved in their trials (the Duke of Brittany and the King of Hungary, respectively) to benefit financially from the prosecutions. In the case of Gilles de Rais, his lands were confiscated by the Duke. In the case of Bathory, Matthias II was able to renege on a substantial debt owed to the countess.

Secondly, some (but not all) of the evidence marshalled against the pair was extracted under torture. These two factors alone make it hard to be certain how much of the case against the pair was exaggerated, or completely invented, by their prosecutors.

Thirdly, a lot of the most widely-circulated, and salacious details that are commonly associated with the pair – such as the idea that Bathory took baths in the blood of her victims in order to retain her beauty and youthful appearance – are first recorded years later in secondary sources of dubious reliability.

I'd add that it's also very unclear what is meant by a term such as "satanism and the occult" in cases such as these. Since there was no such thing as a faith called "Satanism," with its own beliefs and rituals, such terms were defined – if at all – by the Christian church. Definitions varied and could be made to fit the evidence.

Finally, even if it is accepted that de Rais and Bathory were guilty of some or all of the charges brought against them, it's possible to suggest that they had motives other than ritual ones for committing crimes. Sex and power are perhaps more likely to have been involved.