Spoilers ahead for Shazam!
At the beginning of Shazam!, a young Dr. Sivana notices a caterpillar in an enclosure. Later in the film, we see that the enclosure is broken and the caterpillar is gone. And then in the mid-credits scene, we see Sivana scribbling symbols in his cell only to enter into a discussion with that caterpillar, who informs him and us that he is even more powerful than the sins and the wizards that were previously in conflict. It’s a lot to take in from a caterpillar with a voice box, and the caterpillar doesn’t even introduce himself. He’s just a really powerful caterpillar.
If you’re confused about this, you’re not alone. But fans of the Shazam! comics probably recognize the caterpillar as not a caterpillar at all but an alien worm named “Mister Mind”. Mister Mind first appeared way back in August 1943 when Shazam was still called Captain Marvel (don’t get us started on that whole comic book legal drama). Per Wikipedia:
He claimed to be a mutant worm from a distant planet who was more intelligent than the other worms, which communicated by touching their feelers. This Mister Mind possessed limited telepathy and the ability to spin nearly indestructible silk at speeds faster than the human eye could follow, spoke through the use of a “talk box” which hung around his neck which amplified his voice, and was depicted as being myopic and needing eyeglasses. In Shazam #2 (April 1973) he is described as having the body of a lowly worm, the conscience of a Hitler, and the brain of a genius.
In the comics, Mister Mind works through the Monster Society of Evil, a team of supervillains, and one of the first supervillain teams comprised of villains that the hero had fought previously as opposed to villains created just for that story. In the comics, Sivana is frequently a member of the Monster Society, so it’s no surprise that in the post-credits scene for Shazam!, Mister Mind comes to recruit him.
As for what powers a space worm would have, “Mister Mind is one of Earth’s most formidable telepaths. He is able to take control of an individual’s mind. He also possessed many insect-related abilities, like the ability to spin very strong silk at speeds faster than the eye can see, so quickly he can encase a human in a cocoon within seconds.”
So will Shazam be fighting Mister Mind in a sequel? It seems likely. Now that Shazam has his own team comprised of his foster family, it seems like future conflicts will be team battles. It’s easy to see how Mister Mind will compose a team to take on the super family. Whether the Monster Society comes to fruition in Shazam 2 or a little ways down the line remains to be seen.