Seems completely believable. If you're open to such things, you'll think that it's entirely possible for something like this to have actually happened at some point, somewhere. And the fact that it seems so believable just goes to show the stupidity and evilness of the human race. The idiocy behind the witch hunt is historical fact and the idea that someone would summon a being of Lasher's power and cunning just for fun really isn't far fetched at all, so that's the stupidity part. And the evilness is shown throughout the book, seeing how many of the Mayfairs used their power, in Cortland's poisonings, even in the Talamasca, but especially in Carlotta's schemes. These are fully believable; many did such things when they could and, granted such power, many would do all of them. As for Carlotta, her "quest" lacks a point.

Spoiler

Using her power to defend herself is one thing, but arranging to have Stella killed, terrorizing and then being responsible for killing Antha, not killing Deirdre but driving her insane, trying to wipe her brain and keeping her sedated all her life and then sending Rowan away, making her adoptive mother force her never to return to find out her true origins were strategies doomed from the start. A being of Lasher's power can't be stopped this way, so all she did was reveal the evilness of humans in general.

If you ask me, Carlotta's way worse than Lasher; Lasher's just Machiavellian.
Otherwise, the part about the history of the Mayfair witches is a hard but very good read and the action at the end of the book just reveals what happens when people try to do things alone when they could have help and the harm that can be done by hiding thigs from others "to protect them", because they'd be upset if they knew.
As an end note, Rice's fascination with older men, violent sex, old-fashioned behavior and amazing wealth is still there...

As a person who has nothing against abortion, actually thinking it should be mandatory in case of an accidental pregnancy or if there is any risk of the child being born with any health problems, and is very much for stem-cell research, the strong views against such things by some characters in the book really bothered me.