Conflicts of Universal Rules
One problem you may encounter when traveling the Multiverse is being restricted by Physical or Magical Laws that don’t mesh with those of your origin Universe. If you originate from Nexus Prime, this problem will be less pronounced than if you come from anywhere else, because we’re really adaptive here. One of the worst conflictions is coming from a Magically Positive Universe, where magic exists, and going to a Magically Negative Universe, where it doesn’t, and you’re a mage. You will be completely incapable of using your Magic, and you might be weakened physically by it. Alternately, you could enter a Universe who’s Laws make your magic unpredictable and dangerous to use.
Some magical spells or technological devices can allow you to “take a piece” of your home Universe with you, encasing you in a pocket of your own Magical Laws. A Centaur from the Xanth Universe is capable of doing this unconsciously, creating a “corridor” of the Xanth peninsula’s magical atmosphere into the rest of the world, which is Magically Negative. Some of the Brogalios out there can tap into stores of magic from Hammerspace from anywhere in the Multiverse and have it work perfectly regardless of the Magical Laws. I myself don’t dabble much in Magic, although some of what I do in NA could be considered a kind of Magic. But that’s Authority, which may or may not actually be magic. It depends on who you ask.
There are some Universe out there with Physical Laws that don’t match up with others. The most extreme cases are those where the dimensions of “forward-backward,” “left-right,” “up-down,” and even the flow of time are out of alignment or “out of phase” with the norm. Typically, one can’t access these off-beat areas, but anything’s possible with the right kind of explosion, as the Star Trek series have shown. In at least the first two series, there’s an episode where a couple of characters get knocked into an alter-phase Universe sitting alongside their own, rendering them capable of walking through stuff and also invisible.
As far as I can tell, there’s only one way for Time to be skewed from the norm: by flowing in the opposite direction. This can be viewed by visitors two ways: the world seems to be moving in real-time rewind or time seems to flow normally for a few minutes, followed by a strange feeling and distortion of the surroundings as the clock moves backwards about an hour or so. I highly recommend getting out of Universe like that as quickly as possible. They can hurt your head with time paradoxes.
As with magic, there are devices that can adjust you to the altered Physics, chiefly alterations in the strength and direction of Gravity. Unfortunately, there is currently no known way to adjust a visitor’s biochemistry to handle similar but subtly different, and thus potentially lethal, food types, or differences in the atmospheric composition (if you don’t count space suits). You could accomplish this with Magic, but you’d have to undo the spell when you go home or enter a Universe that’s compatible with your origin point.
Even if you’re unaided, you’ll still carry some aspects of your home Universe’s Laws with you, which can give you a distinct advantage or disadvantage. H. P. Lovecraft crafted a dangerous, nightmarish Universe in the early 1900’s, where exist creatures that can drive a man insane by the mere sight of them. The best known of these is Cthulu, a great beast standing tall enough to walk through the Pacific Ocean with its head above the water, and appearing as a humanoid with an octopus-like head and draconic wings. Lovecraft states that seeing even a statue of this beast can drive you clinically insane. Speaking from the viewpoint of Nexus Prime, I refuse to believe that. In my life, I’ve seen three different representations of Cthulu, and I’m still here, sane as ever. I haven’t had the time to prove this, but I think that in Nexus Prime, if something can be described accurately enough to draw a picture of it, it lacks the ability to drive you crazy just by sight, since it can be comprehended by the human mind. So I bet I could stare Cthulu in the face and not flinch. At least, until he steps on me like an ant…