Spider-Man: Far From Home introduces a Mysterio who claims to have come from a parallel reality. While many casual fans more familiar with the 616 Mysterio might rightfully assume this is all an elaborate hoax with insidious goals, fans of the Ultimate Marvel Universe know there might be some basis in the comics for this one.
In Brian Michael Bendis's run of Ultimate Spider-Man, Mysterio comes from another universe. In fact, he comes from... the 616 Universe. That's right. The mainstream Mysterio fans know and love plagued more than one Spider-Man in more than one dimension. But how did Quentin Beck get there? How did Mysterio cross dimensional lines? Before you see Mysterio in Spider-Man: Far From Home, take a look at the first time a Mysterio from one universe crossed into another.
The truth is that Mysterio, in many ways, didn't directly cross over into the Ultimate Universe. Rather, he sent an android through a dimensional portal. This android functioned as an avatar in this other world. His plan was very simple: Enter a universe with a younger, less experienced Spider-Man and use his villainous knowledge and experience to overtake the criminal underworld unimpeded by his now-loathsome rival.
While Mysterio at first committed low-level crimes, he only really kicked it into high gear following the events of Ultimatum. In Ultimatum, Magneto distorts the world's poles to create apocalyptic weather conditions. In the massive conflict, dozens of characters die. Peter Parker happens to be one of the few survivors.
While others saw this as a cataclysmic tragedy, Mysterio saw this as his chance to enact his plans.
The New Kingpin and Spider-Slayer
Following Ultimatum, Mysterio hunted down the biggest crime lord in New York, the Kingpin, and killed him. This established Mysterio as the most dangerous figure in the New York crime scene.
When Peter Parker came to confront Mysterio, the supervillain managed to almost kill Peter by luring him into a trap. First, Mysterio created an illusion of the Hulk (supplemented by real detonations to simulate the damage the Hulk left behind). Once Peter was in range of the Hulk, Mysterio set off dozens of explosions in order to take down Spider-Man.
Had the Shroud (secretly Kitty Pryde) not interfered, Mysterio would've killed the Ultimate Peter Parker. However, the android soon found blood belonging to Peter on his person. He created a Spider-Slayer to follow the genetic scent of Peter's blood, tracking him down to Midtown High School. While the Spider-Slayer was defeated, Mysterio learned Peter's identity.
Mysterio was eventually "defeated" by the combined efforts of Iron Man, Spider-Man and the Black Cat, though Mysterio managed to destroy Iron Man's armor and almost end Black Cat's life.
However, it was only then that Peter realized Mysterio was simply an android being controlled by someone else.
The nature of Mysterio's interdimensional travel wasn't entirely unearthed until the Spider-Men storyline, where 616 Peter Parker tracks down Mysterio in a subterranean bunker. Mysterio, it turned out, has been preoccupied constructing another avatar, and, to Beck's disappointment, missed when Norman Osborn killed the Ultimate Spider-Man. At the present, a new Spider-Man had appeared in the Ultimate Universe, and Mysterio saw this as an ideal time to attempt a takeover once more.
However, 616 Spider-Man ended up finding his way over into the other world, and, with the combined help of Miles Morales, managed to defeat Mysterio once and for all, cutting off Mysterio's access to the other world permanently.
How Did Mysterio Make His Portal?
Mysterio didn't just stumble upon a magical, interdimensional portal. He constructed a machine that punctured a hole to another reality. This took an unbelievable amount of technological skill and energy. The machine's energy signature led to 616 Spidey finding Beck's hidden base. It allowed him to send his mechanical avatar into another world. However, the extent of Mysterio's technology remained beyond understanding.
Ultimate Tony Stark, after spending about a day examining the interdimensional technology and Mysterio's mechanical avatar, fails to figure out how Beck's technology functions or how he came across it. It is highly possible that Mysterio designed it. The man is incredibly intelligent, as seen through his increasingly elaborate technological development. That also said, he could have just as easily stolen all the tech.
Mysterio Was Still in the 616 Universe
It is important to note that Mysterio never really left the 616 Universe. Rather than leaving one world in favor of another, Mysterio's antics in the Ultimate Universe were more a side adventure for him. He still took part in several events in the 616 Universe. Most notably, he participated in the Resurrection of Kraven the Hunter and Doc Ock's Ends of the Earth plot.
In fact, during the Spider-Men storyline, Mysterio for a moment considers the events of the arc a triumph for him, as, for awhile, it appeared as if he had trapped the 616 Spider-Man in the Ultimate Universe, granting him a world without any Spider-Men.
This proved not to be the case. However, for a brief period of time, Mysterio thought he might've won.