The First Breaking
The First Breaking causes cracks in reality itself, letting some beings in from other parts of the multiverse before sealed.
Long before anyone started to bother with counting years, reality cracked. Something from outside of the whole of creation tore through all the planes at once. The rip was deepest on the Astral Plane, but every plane was affected in one way or another. In the Feywild and the material plane, elves entered the world. In the Astral Sea, the Illithid empire was waging war against the Gith. And everywhere, boundaries were torn.
The most popular theory among scholars is that the cascading effect set in motion by the creation of Shar
reached its limits. When the twin goddesses created the ripples that pushed order to one side and disorder to the other, the light concentrated in another area, opposed by darkness. These two sets of polar opposites shaped the planes as they came to be. But, it might be that they eventually stretched the worlds to thin and caused the Breaking.
Another theory suggests that someone amongst those who appeared was experimenting in a different world and crashed into this reality. Someone in the city of Sigil
is a popular thing to point to as that plane was caught half-way in the Breaking and torn asunder as the crack closed. Or the crack closed because Sigil was destroyed, depending on which theory you're listening to. Those who believe in this actor from the outside speculate wildly as yo who it would be and why they would do it. very soon, they tend to end up with various conspiracy theories.
Not everything that entered during the Breaking did so into an existing plane. Sometimes a small pocket-dimension, known as a Reality Shard, got stuck and is not a part of one of the regular planes or floating around in the Astral Sea. Some of them it's possible to slip into by accident and getting out might be much harder. Some carry beings that have been in stasis since the breaking. Almost all of them are mysteries and unexplored.
The rip, in reality, ended a few months after it had first opened and closed as suddenly as it had opened. The people and places next to the closing tear were either sucked in or changed forever. There are various ruins or strange phenomena where this happened.
It isn't known why the First Breaking ended, but some have noted that the planes seem to not expand in the same way since. Planar travel became slightly more difficult during this time and didn't recover by the time the Second Breaking
blocked it permanently for some.