The Masters of Evil! The Brotherhood of Evil! The Brotherhood of Evil Mutants! The Injustice Gang! It's not at all uncommon for super teams to have arrayed against them a complement of villains who have joined forces to defeat the defenders of justice. But for whatever reason it took the Fantastic Four nearly forty issues of their comic before such a gang raised its vile head. (If you don't count the Red Ghost and his trio of Super-Apes and I don't.)
The Frightful Four were organized by the Wizard to face off against the Fab 4. To that end he associated himself with Paste-Pot Pete (redubbing himself the "Trapster" since even he realize "Paste-Pot Pete" was a seriously stupid name) and Sandman (from the pages of Spider-Man). To make a foursome the team made a pact with Madame Medusa, a truly mysterious woman who would soon reveal herself to be an Inhuman and fall away from her villainous partners. But where did the Frightful Four come from? As it turns out, it's a very...ahem...strange tale.
Specifically it was in the pages of Strange Tales when Marvel attempted to break out the Human Torch into his own solo feature. He needed villains and Marvel complied with the Wizard. The Wizard was a genius who grew bored and turned to crime to make money and demonstrate his superiority and disdain for his fellow man. The Human Torch defeated him.
Then came Paste-Pot Pete a few issues later, a disgruntled artist who made use of a literal paste pot and an attached gun to strike out on a career of crime. This absurd figure was also soon defeated.
The Wizard turns up again a few issues later and takes on the Torch all over again, now an enmity between them has taken on a personal quality.
Then it dawns on the Wizard that he might need allies and he makes a partnership with Paste-Pot Pete in jail and the two escape, only to very quickly strike out in vengeance against the Human Torch. But it's a rough and tough partnership and ends in yet more failure.
Enter the Sandman. The Sandman debuted in Spider-Man but soon enough is battling the Torch in the pages of Strange Tales. With relative speed three of the future Frightful Four have taken a swing at at least one member of the Fantastic Four.
Paste-Pot Pete is somewhat less ridiculous as he by himself battles both the Human Torch and the Thing. One member of the Frightful Four battles two members of the Fantastic Four -- a foreshadowing perhaps.
Then in the pages of Fantastic Four #36 the Frightful Four finally arrive. The two former allies the Wizard and Paste-Pot Pete (who adopts the name of Trapster in this issue) join with the Sandman and the alluring Madame Medusa to attack the entire Fantastic Four in their Baxter Building headquarters. It's a bold move by a quartet of villains who have some surprising oomph.
Sandman shows up in a solo gig at first and later as partner to Blastaar the Living Bomburst. These were among the very first FF comics I ever saw (thanks to an older cousin) and have an outsized impression in my memory. Reading them later I still like these two comics and have always liked Sandman's costume though he rarely used in subsequent years.
The Wizard shows up in a couple of issues in which he tackles the Thing, or more properly Ben Grimm since this was a period when Ben was inbetween transformations. Despite debuting his "Wonder Gloves" the Wizard falls short once again.
During the Lee-Kirby years the full foursome show up in a single spooky issue to battle the Fab 4 and as it turns out end up taking on a witch. This is the comic that featured the debut of Agatha Harkness, who would become the supernatural nanny for little little Franklin Richards and still later a mentor to the Scarlet Witch in the pages of the Avengers.
And that's a wrap for the Frightful Four. They show up several times over the decades, the core trio of Wizard, Trapster, and Sandman finding a fourth partner in the likes of Thundra and the Brute. Sadly they were rarely "Frightful" and in many appearances they weren't even "Four", so their name was an ongoing disappointment many times.