The Batman Who Laughs #1 by Scott Snyder

Investigating a mysterious corpse that proves to be an alternate Bruce Wayne, Batman discovers that a sadistic counterpart has returned to Gotham with a plan that weaponizes versions of him from across the Dark Multiverse. As more bodies surface and a heavily armed, gun-toting Dark Knight stalks the shadows, the case tightens into a trap that pushes him to his limits. In a shocking turn, the Joker infiltrates the Batcave and unleashes a death-triggered toxin designed to corrupt Batman from within, setting the stage for a battle for his very soul.

Purchase from Bookshop.org