The Grimoire du Merlock is a spellbook.
A relatively small dark-green tome, the oddly-titled Grimoire du Merlock is a spellbook in which immortal dark sorcerer Merlock consigned his knowledge of a number of spells, both good and evil. At some point, this book (or, assuming it was ever more than a manuscript, a copy thereof) ended up in the possession of Webby Vanderquack, who sneaked it into McDuck Manor despite Scrooge McDuck's ban on spellbooks in his house. In 2018, Webby found a spell based on friendship and a prized possession which she used (along with her friend Lena) to deactivate Tiffany, Magica De Spell's shark-like money golem.
Behind the scenes
Beyond the already-puzzling mix of English and French in the title, the French part is not actually grammatically accurate: assuming the intended reading is “the spellbook of Merlock”, then it should be “Grimoire de Merlock”, as opposed to “du Merlock”. “Grimoire du Merlock” instead translates to “spellbook of the Merlock”.