From publisher blurb:
While most spirits shy away from the noise of cities and towns, the Magpie revels in their splendor - so many things for the taking! The Magpie is a spirit whose goal is to collect the truly shiniest of things to add to its collection. It almost solely judges its collection on each item's luster - the Magpie could collect something precious, like a sparkling gemstone, or trash, such as a loose scrap of metal which catches the light, and value both equally.
Opportunistic warlocks may make pacts with The Magpie aiming to collect things which would otherwise go unappreciated by their patron, or simply find satisfaction in delivering interesting baubles to the generally benevolent spirit. Whatever the case, the Magpie seems to have little interest in the motivations of its warlocks, so long as they remember to whom the shiniest prizes belong.
Building off of the existing Fifth Edition rules to build a warlock character, the Magpie grants warlocks unique abilities revolving around collecting cheap trinkets and baubles.