Necronomicurse is received when you get the Necronomicon from the Cursed Tome event. This curse cannot be removed from your deck by any means and any attempt to do so will return the card to your hand/deck.
The Necronomicurse counts for Du-Vu Doll, and can be used to trigger Immolate damage (although the card will be returned to your deck rather than Exhausted).
Omamori will negate the Necronomicurse entirely, but only if Omamori is acquired first.