But the roots of the whole idea actually go back to the Ultimate Universe! A few years into the alternate reality's existence, fans wondered when and if these new versions would meet the classic characters they were based on. So, when Mark Millar and Greg Land launched the "Crossover" story that ran in ULTIMATE FANTASTIC FOUR (2003) #21-23, people got excited about the possibilities, especially when it seemed like the original Reed was communicating with the younger version.
Instead of a trip to Earth-616, though, the Ultimate FF opened a gateway to Earth-2149...which was inhabited by zombies! For them, it all started when a zombified Sentry crashed to Earth and began spreading the virus around to the Avengers, X-Men and, of course, the Fantastic Four.
The zombie FF wanted to open a portal to other worlds so they could eat, having turned just about everyone in their reality in three days! As Reed allied himself with that world's Magneto, his teammates captured their zombie counterparts before crossing over to help fight an army of zombie heroes. Magneto allowed the Ultimate FF to go home while destroying the gateway receiver on his end.
The following year, readers returned to the flesh fiend world with MARVEL ZOMBIES. The five-issue series picked right up where the previous story left off, with Magneto standing up to the undead heroes. He put up a grand effort, but ultimately fell. The former heroes then focused on finding their next meal...which presented itself in the form of the Silver Surfer! The infected took on some of his Power Cosmic when they ate the Surfer—which helped them take down Galactus not long after. Granted portions of his power, the surviving zombies took to space to find new planets to feast upon.