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...