When an ancient spell book activates a cursed family heirloom, a part-time occult librarian teams up with an undead ex-cop to destroy a voodoo witchdoctor stealing souls to live forever.

Excellent, that works.

I could probably get rid of 'dead' too, since the feeling of 'body' in this context could be considered 'without life'.

Which brings it back to a more comfortable 28 words...

'When her mother's body is stolen, a family heirloom links an occult librarian with an undead ex-cop to destroy the thief, a voodoo witchdoctor trying to live forever.'

If you change "goes missing" to "is stolen" then "the person responsible" can be "thief" saves you a few words. Word count I don't care to much about. If the story works it works. I have a real sense of your story now.

We'll give this a shot...

When her dead mother's body goes missing, a family heirloom links an occult librarian with an undead ex-cop to destroy the person responsible, a voodoo witchdoctor trying to live forever.

Right on 30 words, which is disappointing, but that damn pronoun game gets me every time...

Perhaps giving the book and/or the heirloom more of a sense of benevolence, I think I got stuck on how much better "cursed family heirloom" sounded without thinking whether or not "cursed" can sometimes be seen as a positive attribute - the Genie in Aladdin is cursed to live his life forever in the lamp, but he has limitless power - that kind of thing.

The way I see it is the book and heirloom are merely tools that the nature of the possessor gives the aspect of a positive or negative thing - ergo, the witchdoctor is the antagonist.

However, since there is confusion to be had, I'll concede the point. Especially since there does look to be a reason for the goal that is missing from the current logline as kbfilmworks said above.

Your logline starts off 'when and ancient spell book activates a family heirloom'

So I am thinking something like 'Todd and the book of pure evil'

Then later in the logline you switch to a witch doctor trying to steal souls.

That seems confusing, as a result I would just stick with the witch doctor; 'when a witch doctor activates a family heirloom'

How is the book an antagonist? I really don't understand how it could be, ever. But how in this instance?