Look and marvel at Pete Woods and Paul Cornell's Mister Mind!

They say a hero is only as good as his villains, which is one of the reasons I think Captain Marvel is one of the greatest superheroes ever—he's got the best villains! In addition to his archenemy Doctor Thaddeus Bodog Sivana, he's got Mister Mind, an evil alien worm determined to conquer and rule the world.

He was first introduced in Fawcett's Golden Age Captain Marvel comics, as the unseen mastermind behind the Monster Society of Evil that was bedeviling Cap and the whole world. The revelation that he was just a little worm, who wore glasses to see and a little radio around his neck to talk, was a surprise twist. He was put to death in a tiny little electric chair, mounted atop a regular electric chair, and then stuffed and mounted.

But he returned, repeatedly!

After DC bought Fawcett and absorbed their characters, Mr. Mind was relegated to Earth-2 and Earth-S, but was introduced into the post-Crisis DCU in a short-lived miniseries it's probably best to pretend never happened (in that, he was the worm in the bottom of a tequila bottle Sivana was drinking on) and then better and more thoroughly by Jerry Ordway in his Power of Shazam series. Ordway's Mister Mind was a Venusian evil alien worm determined to conquer and rule the world, the vanguard agent of a race of Venusian worms. Ordway's was less cartoonish and more realistic in appearance, looking much more like a caterpillar.

Mind played a big and surprising role in the weekly series 52, during which he had grown from his larval form into a "Hyperfly," an evil butterfly-like cosmic creature that feeds on entire universes.

Writer Paul Cornell and artist Pete Woods brought Mind back in another surprising appearance during their Action Comics story arc, "The Black Ring" (reviewed here).

Mind appears in a splash panel on the last page of the first issue (#890), emerging from the head of one of a trio of mind-controlled kidnappers who have attempted to capture Lex Luthor:
As you can see, Cornell and Woods' Mind looks an awful lot like Ordway's, although now he's bigger—about the size of a large stuffed animal—and while his body is rounder and fatter, his legs and mandibles are pointer and more menacing looking—sometimes. Depending on the context. Here he's dripping with gore, and meant to be a bit scary.

He still talks via a radio or "talk box," although it's a little more modern and space-age in design. While the original wore spectacles, and Ordway's had big, round eyes that visually echoed the original's glasses, this one was the segmented eyes of an insect, save with one in the middle being black, suggesting a pupil. The result is an eye that looks insect-like and cartoonish at once.

Cornell usage of Mind is as the first of many villains Luthor is juxtaposed with throughout the year-long storyline. Mind is working for someone else, although he himself doesn't know who or what he's working for. He (and we) just know it's something incredibly powerful, so powerful its made a lackey out of a villain who has organized Monster Societies, attempted to conquer the world and once almost ate the whole Multiverse.

He engages Luthor in a sort of mental combat, psychically imprisoning Luthor in a series of absurd fantasy sequences, including ones in which Luthor is a Promethean caveman stealing fire from super-gods, another in which he is a playing the role of Doctor Frankenstein from the James Whale movies, another in which he's a Godzilla-like giant monster and Luthor is a Superman-like superhero and, most adorably, a wild west sequence in which Mind wears an adorable cowboy hat and wields a shooting iron:

 

That gives you a good idea of this Mister Mind's size, and how having him emerge from your head would be the end of you.

Unfortunately, it also means he's not much for hand-to-hand combat. This sequence shows what he did to the poor sap whose skull he was occupying (note the prone, headless body in the first panel), and how easily Luthor defeats him once they've escaped the mental plane for the real world:

As to how he returned after his time as a Hyperfly in 52, he attempts to explain to Luthor that he's actually the offspring of the original Mister Mind, but "our consciousness is passed down on a strand of eight-D R.N.A."

He returns again before the story ends (in a chapter drawn by Jesus Merino), when his unseen master is finally revealed and their relationship can be explained without really spoiling anything.

He appears in outer space alongside Lex, the "Lois-bot", the cosmic entity and Superman in the climax of "The Black Ring", although he's not really there-there, but sends a "four-dimensional holographic projection" of himself...meaning either Mister Mind survived being punted off a building or, just as likely, reproduced a "grandson" of the original Mister Mind in the same manner that the new Mister Mind at the beginning of the arc was created and shared the consciousness of the original.