Warning: Spoilers for Shazam are ahead.
At the end of Shazam, Billy Batson (Asher Angel/Zachary Levi) and his newly super-powered foster siblings teamed up to beat villain Dr. Thaddeus Sivana (Mark Strong), who was imbued with the strength of the seven deadly sins. After they saved the day, they banished the demons back to the Rock of Eternity (a.k.a. the wizard Shazam's cave-like lair) and Sivana headed to prison. After that came the Shazam post credits scene that might be tough to follow if you aren't big into DC Comics. Luckily, we've got you.
In case you missed it, following a gorgeously animated end credits sequence, the screen went back to a shot of Strong's Silva frantically scribbling on the walls of a prison cell as he hears a voice. When he looks around, he realizes it isn't another person who is talking to him, but rather a tiny green worm perched on his windowsill.
OK, So Who Is This Worm In Shazam?
No, you did not hallucinate. The talking worm is actually a character that dates back to the original comics on which Shazam is based, and was introduced in 1943. The worm's name is Mister Mind, an alien with the body of a worm and the mind of a super-genius. If you paid close attention to the scenes set at the Rock of Eternity, you might've noticed a little caterpillar-looking bug in a jar — and that later, the jar was shattered.
Cool. So What Does This Worm... Do?
Mister Mind founded the supervillain team called the Monster Society of Evil, which was also comprised of Sivana, Captain Nazi, monster Hydra, super-strong Ibac, and more characters, along with real-life evil Axis leaders Adolf Hitler, Benito Mussolini, and Hideko Tojo (all in power during the character's World War II-era inception).
While he was always a super-smart alien worm, the character's backstory has evolved to evolution established its origin as a race of telepathic worms with the power to bend space and time. The character was reintroduced in DC Comics' New 52 series — which rebooted the timelines of every single DC character thanks to actions by The Flash — by former DC Entertainment CCO Geoff Johns. The film drew plenty of inspiration from that iteration, including the origins of Billy Batson and his foster siblings-turned-superheroes.
So... Are We Getting A Worm, Um, Movie?
The final frame of that comic saw a caterpillar introducing himself to Sivana and saying that they were about to become the "best of friends," potentially foreshadowing the two characters teaming up for a new version of the Monster Society of Evil (and, likely, a yet-to-be-officially-announced Shazam sequel).
In the film, the little guy told Sivana, "the seven realms are about to be ours."
While a Shazam 2 has not yet been officially greenlit, there's definitely more Shazam stories in the works. The film was originally conceived with Shazam battling the villain Black Adam, who would be played by The Rock — but Black Adam's story was spun off into his own still-in-development origin film. After the opening weekend success of Shazam, it's unclear whether the next film from the Shazam world will be a sequel featuring our little worm friend, or a major spinoff.
Either way, this little worm clearly did what he came to do.