https://www.cnet.com/news/shazam-dc-movie-postcredits-scenes-explained/

Early in the movie, sharp-eyed viewers will spot a mysterious worm in a jar at the Rock of Eternity. We later see the jar cracked and empty, and there's no sign of the strange worm as Shazam (Zachary Levi) and his friends take down Dr. Sivana (Mark Strong) and the Seven Deadly Sins.

However, midway through the credits, we cut to an imprisoned Sivana scrawling on the wall of his cell. He's trying and failing to regain his lost powers, until he hears a mocking voice from the window.