Stolas's Grimoire

Stolas's Grimoire is a arcane book belonging to Stolas used by the I.M.P. company to illegally access the living world.

Background
The exact nature of the Grimoire isn't clear, but Stolas was entrusted with it to carry out unknown duties in Hell, and one of its abilities is to allow the opening of portals between Hell and the living world.

Eventually it came into the possession of Blitzo, I.M.P.'s founder, where it became an essential part of the company's business model of travelling to the living world to assassinate targets on behalf of souls damned to Hell. How exactly Blitzo came to have the Grimoire is unclear, as in the series pilot it's shown that Blitzo stole the book off Stolas after sleeping with him, while in the first episode "Murder Family" Stolas implies he lent it to Blitzo.