The Convergence

There are four cardinal Ideals (Law, Chaos, Good, Evil). There are four fundamental Elements (Fire, Air, Earth, Water). So why are the inner planes of the cosmos a jumbled mishmash of three imperfectly overlapping planes? Cosmology doesn't work that way.
The answer is that before the Convergence, there were four inner planes surrounding the Material Plane, between it and the Elemental Planes.

by M

The base Substances: Life and Death, Magic and Mundane. At the center, the Material Plane was the perfect balance between them all, and the only truly complete plane in the universe. The Inner planes were all reflections of the Material Plane that incorporated parts of their neighbors but almost nothing of their opposites.
The Life Plane was the source of energy that went into all mortal life in the universe. There are beings (gods, archfiends, the Lady of Pain, and some others) that transcended or preceded Life, but they are not technically "alive" in any sense we would recognize, despite possessing sapience and will. A tremendous number of extant species originated here, most notably the elves. This explains why elves are almost uniquely capable of adapting to whatever environment that find themselves in, leading to more subspecies than any other species by far. Virtually every animalistic humanoid species first arose in the Life Plane as well, such as Harengon, Leonin, Tortles, Kenku, and many many more. Life is oriented toward the Good Ideal, reflected in constant creation and growth. The Life Plane balanced the magical and the mundane but contained absolutely no death or decay. If anything happened that would have killed something or someone, they instead simply faded out of existence.
The Plane of Death, on the other hand, contained absolutley nothing living. The "native species" of the Death plane were skeletons and zombies, resembling humans or elves but having come into existence already dead. Like the other Inner planes, Death was a reflection of the Material Plane, but in a perpetual state of decay. The Shadowfell is far less drastic now than the Death Plane was originally, and the endlessly adaptable elves have even begun living there (amongst others), but that would not have been possible before the Convergence. Today, it is possible to use Necromantic magic to raise skeletons and zombies without targeting an existing corpse, by summoning those beings directly from the Shadowfell.
The Plane of Magic was the wellspring of power that drives the universe. It was the original home of the dragons and their lesser kin the kobolds. (I'm undecided about Dragonborn.) Everything there was magical, completely free of any mundanity. It is hard for us to imagine exactly what that could have been like, where such things as breathing or gravity worked through magic and never as simple facts of existence. Oriented in the direction of Law, magic is deeply legalistic and follows strict rules under most circumstances. It is not free of the influence of Chaos however, and this comes out in bursts of wild magic and other unexpected effects. Life and Death were balanced, and magic is equally capable of healing and killing. (It is a reflection of the nature of Adventurers, and not of magic itself, that destructive spells are far more common in use today.)
The Mundane Plane had no connection to magic. This made it unique amongst all the planes in the universe, since magic is the only way to travel between the planes. Any attempt to use magic to enter the Mundane plane inevitably failed, since magic is needed on both ends of any portal or gateway in order to safely emerge. (Was Sigil the only exception? Was there a way into the Mundane Plane here, but you would instantly lose your magic if you crossed through, and thus could never return unless the door just happened to be opened from the other side?) As a result, the Mundane plane was seemingly ignored by the rest of the universe. Like every plane, there were sapient humanoids that lived there, but in isolation, they developed differently from all the other planes. At first there were many different species, just like elsewhere, including the equivalents of humans, orcs, elves, halflings, and more. But eventually, humans outcompeted and exterminated every other humanoid species, then set about conquering and subjugating their entire world. Without the tempering influence of magic, humans went mad collectively. They knew that gold and other precious metals were valuable, but had no idea why, so they dedicated their entire existence to collecting them. They struggled to perform basic tasks without magic, and developed technology for every contingency, and then developed it further to solve things that were never a problem to begin with.
This all ended when an evil god found a way to pierce the bubble and invade the Mundane plane, retaining its magic while there. If there is any being in the universe now who knows how this was accomplished, they are not talking. (As the creator outside of the universe, I can say roughly: the god invaded Magic first, and tied its heart to that plane in a way that allowed it to directly funnel raw magic. It projected this magic at the "bubble" of the Mundane plane, piercing it. The god's goal was to recruit an army unlike any other in the universe, and use it to invade the Upper planes.) The humans could make no sense of this being, torn between believing that it was a supernatural god, an ancient kaiju, and alien invader, or any number of other ideas. At first the god tore through the plane, seemingly unstoppable. But humans should not have been underestimated: a lack of magic does not mean they were completely powerless. High tech weaponry proved effective against the god, and the ultimate weapon succeeded in killing it: nuclear bombs.
Through chance, fate, or the dying will of the god itself, the god's heart essentially fell into the lap of one of the worst possible people to find it. This person was one of the principle proponents of the 'we are being attacked by a supernatural god' theory, and had quite the god complex himself. He used the heart to enact what was more or less the equivalent of a 10th-level Wish spell. He wanted to be the savior of humanity, to unite everyone under him, to bring magic to the world, and to change history to fix the destruction caused by the god's incursion.
While he happened to have guessed correctly about the god and about the existence of magic, he was completely ignorant about the metaphysics of the planes. His Wish was the specific cause of The Convergence. The tether that the god's heart had to the Magic plane caused it and the Mundane plane to hurtle toward each other, crashing into and merging with the Material plane. The metaphysical vacuum created by this sucked in the Life and Death planes as well, though to a lesser extent since they were not directly targeted. The humans of the Mundane plane, as well as the dragons, kobolds, and other beings living on the Magic plane, were deposited on the Material plane. Both Magic and Mundanity became much stronger influences there then they had been. Critically, reality was rewritten so that things had "always" been this way: as far as anyone could remember, there had always been humans mixed in with all the other races. This new reality is what we now know as the Prime Material plane.
Reality was rewritten, but the change was far from perfect. The high technology, dense cities, and other hallmarks of the Mundane plane don't fit this new reality. But traces remain as ancient ruins with complex traps that somehow are still intact; occasional coins that did not originate in a dragon's hoard; and an unconscious memory among humans of how things used to be ("Mandyisms"). The Ethereal Plane is a remnant of the Mundane plane overlaid on the Material. Similarly, the Magic plane's presence can be seen through the existence of magic items too powerful for anyone to replicate; stronger magic now than anyone remembers in "the old days"; and the ubiquity of dragons with their direct connection to magic itself. The Astral Plane is the remnant of Magic, suffusing not only the Material but extending out to all the planes just as Magic once flowed everywhere. The planes of Life and Death were not completely absorbed into the Prime Material, but are much more closely linked to it now than they used to be, existing now as mere echoes. Death is now possible in the Feywild, and life can exist in the Shadowfell.
Humans on the Mundane plane had not lived in balance with their environment the way those of the original Material plane had. As a result, there are now about as many humans as there are all other humanoid races combined. While everyone remembers things as "always" having been this way, the peaceful balance that endured for millenia is antithetical to human nature. In the 30 or so years since the Convergence, humanity has begun spreading, seeking power through economic, political, and military conquest.
Among mortals on the Material Plane, the only one who knows that the Convergence happened is Kem Lunos. In the Feywild and Shadowfell, the most powerful beings have a memory of a dream of how the world used to be; for all others, their memories (and records and whatnot) have been completely rewritten. On the elemental planes, it is known that a catastrophe befell the material plane but details are vague. Only the most powerful know exactly what is different. The Magic plane was seldom accessed directly, and the Mundane plane never was, so their focus is mostly on how the Material plane has changed. Aberrations that were not in our universe when the Convergence occurred were unaffected by it, and if they have been here before and return after, will be either baffled or fascinated by the changes.
Meanwhile, the Outer Planes are collectively holding their breath. No one could have possibly predicted that pathetic little humans of the Mundane plane would pose a threat to someone with magic. But they killed a god. And now those same humans are running rampant across the Prime Material plane, the most important realm in existence. They have access to magic, and are becoming aware of the other planes. The various gods and fiends are terrified, though not a single one of them would ever admit it. They collectively agreed thousands of years ago that the Material plane was off-limits to their kind; they could communicate and influence, but never go there in person. Could that be about to change?