Although Reed Richards is one of the smartest minds in the Marvel universe, he is still human and thus capable of being deceived or manipulated. In the Ultimate Marvel universe, the teenage Reed Richards enjoys his status as a member of the Fantastic Four but almost dooms his world and more when he was tricked into opening a portal to another world, the world of Marvel Zombies!
In the 'Crossover' storyline by writer Mark Millar and artist Greg Land, Reed Richards receives signals hailing from another dimension which appears to be a parallel world that is almost exactly like his. Reed is fascinated with this discovery, especially with his conversations with someone who appears to be an older Reed Richards from the mainstream Marvel universe. Under the impression that he is being recruited along with other geniuses from throughout the multiverse for a mission, Ultimate Reed is provided with blueprints for a teleporter that would bring him to this new world. But his joy turns into horror when Reed learns that the world in question is an empty and rotting planet full of superhero zombies, including an intelligent but undead Fantastic Four.
The zombie Reed tells his Ultimate counterpart how a superhero from another world, revealed in flashbacks to be an undead Sentry, appeared one day and infected the Avengers and eventually the entire world was either eaten or converted into hungry zombies. The transmission the younger Reed picked up was a ploy, as they had run out of food and desperately wanted to find another healthy world to feast and corrupt. The zombie Fantastic Four tried and failed to eat Reed and although he escaped, he discovered there were few places that weren't crawling with the Marvel undead. Saved by an uninfected version of Magneto and brought to a distant sanctuary, Reed tells him of the Four's deception and is unfortunately told that his world is most likely under siege from the teleporting undead and soon will be a mirror image of this world.
Millar cleverly tricked the readers into believing the Ultimate universe, which he helped create along with Brian Michael Bendis, was crossing over with the mainstream Marvel universe and instead created something entirely new. Although the Ultimate Fantastic Four was able to defeat their zombie doppelgangers and return home with Reed, the crossover's reaction was so positive that a spin-off series titled Marvel Zombies was announced, picking up right where this story-line ended and the rest is history.
While the zombie Fantastic Four briefly became the Ultimate incarnation of the Frightful Four, the Marvel Zombies story was far from over as it became a successful comics franchise that is still being explored and reinterpreted to this day. Hopefully Reed Richards replaces some of those science textbooks with horror movies, seeing that his naivety and unpreparedness almost brought the apocalypse by opening a portal to an undead hell.