Qfyilyi

Say an explorer lands on a previously undiscovered land, filled with humans, humanoids, wildlife, elementals, undead, demons... you name it.

One would expect that these existing inhabitants would have their own power struggle. For example, the humans would be fending off the undead while also fighting against other humans over resources and hunting wildlife for food; the demons would be fighting against the elementals in a struggle to control points imbued with powerful magicks; the wildlife would have their own fights over food and territory, while the most powerful of them raid human encampments for their food, and so on.

And yet, as soon as the explorer shows up, all these beings seem to immediately forget their own struggles; instead, they work together in an effort to kill the explorer. Indeed, opposing tribes of humans fight together as one, leading the ferocious beasts they once feared and even the undead into battle, while diametrically opposed elementals and demons aid each other in battle, such as a flame demon imbueing a stone elemental with the power of fire.

Aside from the cop-out explanations such as "they are all under the influence of a higher being", how can this behaviour be explained?

He's the Harbinger of Doom

For some particular reason, this character is bringing the end times with him (chasly from UK's answer fits in here).

There's infinite ground to cover here. As a bonus, you can also create a character that ranges from a lawful good guy who, unknowingly, is bringing the end times (or even trigger them by accident) to a chaotic evil one that's actually trying hard to screw every living being in the planet (for motives of revenge, self preservation, power or just simply being an asshole).

He's Cursed

The great Wizardius is a very powerful mage known for casting the most despicable curses upon those who mess with him - and our hero had the bad luck of stepping on his favourite pet guinea pig.

As punishment, Wizardius banished him to another world/land. Shortly after his arrival, he discovered that (nearly) everyone in that place wants his head.

He is told by Unimportant Jenny that there's a spell (to which she's immune) that makes all living things that get near him get insanely filled with rage towards him.

He's in a Simulated Reality

Just picture the Matrix. Here, the character might or might not know he's in a simulation (if you ask me, things are much more interesting if he does but has to play by the simulation's rules to meet his goal).

Maybe our hero was trying to destroy this evil AI, but in order to do so, he had to get inside its own private world to beat it in its own game.

This is a good explanation as to why there's traps and various obstacles in his way that are not people/creatures (traps, mazes, etc).

People just hate his guts

Maybe our hero was once a very, VERY, bad ruler - heads, spikes, walls bad. Or maybe he's the heir of such ruler, a menace that terrorized the entire land for decades. When the opposing forces of the resistance finally got to overthrow this regime, they made it their personal goal to completely exterminate every member of the royal family.

Maybe they have golden shiny hair or some other unmistakable trait (again, chasly mentioned something like this) which is the reason why everyone knows who he is.

The point is that this guy is, did or was framed of something bad - and needs to run for his life, which would turn your game in a kind of hack'n'slash-survival thing.

He is fulfilling a prophecy. Maybe he has a different coloured skin from everyone else or he has unusual eyes. Maybe he wears a particular set of clothes or carries a particular weapon. Maybe he has some technology they don't know about that looks like something prophesied.

The prophesies predict his appearance and state that he will come and break the world and yet, at the same time unite it.

They don't want either.

The explorer came from an area that everyone knows that is forbidden to get in and out. Whether be a cave or a desert, the inhabitants knew that ever since that area existed, no one had gotten out of it and thus forbid anyone from coming near it. An unknown creature exiting out this forbidden area, while the inhabitants are curious, they are also scared as it's possible that the explorer was the cause of the disappearance.

Another possibility: The inhabitants have evolved to have some emphatic ability, able to identify another inhabitant with their own "feelings". The explorer does not have this ability. Upon showing up, the inhabitants, when using their emphatic abilities, fail to feel anything from the explorer. Threatened by the unknown being that feels like an inanimate object, they attacked the explorer.

Despite the fact this explorer is your protagonist, a common motivation for enemies to band together is if their shared land/world/place is threatened. This explorer has arrived and somehow begins to destroy/overpower/infect the world in which all these other characters inhabit. Perhaps he brings a virus/bacteria that their biology isn't immune to. Perhaps, like chasly from UK mentioned, he is fulfilling something that requires the destruction of their land.

Your character should pose some sort of threat. These inhabitants of the land would undoubtedly forget their usual tiffs and work together to save their home.

In Harry Harrison's "Deathworld", this happens - all fauna on one planet is deadly aggressive toward outsiders. Toward the end of the book we find out why (based on my recollection):

the animals are psychic and can read the emotional state of the settlers, who don't belong there, and who feel afraid of the wildlife. The animals' proactive aggressive attitude is something of a learned defense mechanism and is not planetwide, but rather has worsened to an extreme near humans' settlements.

On your world it could be possible that the animals (be they smart or stupid) are psychic toward the emotional state of other animals, and intelligent off-planet outsiders stick out like sore thumbs to them and/or present as threats to the animals' existence. (Perhaps aliens have a "blank" or "hidden reading", and to the natives this is terrifying.) It would not be hard to suppose that as this trait evolved from a common ancestor, the default instinctual behavior toward aliens came with it and subtantially all animals on the planet will attack on mind-sight.

You can make the distinction that these inhabitants aren't exactly normal humans, wildlife, etc. in a normal land, and are instead are all living in a banshied realm akin to purgatory (which would probably make it easier to exlain the demons and elementals anyway). All beings in this realm would have one thing in common, they were stripped of their souls when they were sent there.

When your explorer lands here bearing the only soul in the realm, it is certain to draw attention. The eminating lifeforce acts like an ultrasonic beacon drawing all of this lands inhabitants toward the source like zombies toward a sound. Whether these beings desire to have the soul, or find it's presense unbearable, they must destory the person who posseses it to release it.

The explorer didn't go to any other world. He stays here on Earth all the time, he's just under the influence of Bath salts