MeteorDiablo 3 cinematic trailer from WWI 2008 we get to see glimpses of what is happening twenty years after the destruction of the Worldstone and the defeat of the Prime Evils. A blazing meteor is seen falling across the sky, crashing down into the Tristram Cathedral.
We know from the cinematic and other art that Leah and Deckard Cain were at the cathedral at the time the meteor struck, crashing through many levels down into the catacombs below. Blizzard has said the catacombs we see here are essentially the same ones we visited in Diablo I, where Diablo himself was once defeated.
In the BlizzCon 2008 playable demo, Captain Rumford says a meteor had struck the cathedral and it has awakened evil within. He asks you to investigate. This awakened evil turns out to be King Leoric, or possibly something deep underneath, where the demo characters were not able to proceed to. If this will be the same in the full game is unknown.
Cinematic Meteor Event
What the meteor actually contains is unknown. The only things fans have been able to speculate about are information given in the concept art.
It is of course possible the flaming stone is just some spirit from above that resurrects Leoric, but knowing Blizzard's intricate story lines, it's likely something much greater than just the Skeleton King.
The most popular line of thought is that the comet in fact is an Angel, or even an Archangel expelled from the High Heavens for unknown reasons and that hitting Tristram Cathedral is either a sign to call the hero characters there, or random chance. The reason for this line of belief is the concept art with angel-like appearance. The two most popular candidates are Tyrael and Izual.
These are the concept art pieces that have been speculated to be related:
Was Cain originally supposed to be in the Cathedral, or was he just not visible in the cinematic?
Meteor impacts, but does not hurt Leah.
Presumably Leah looks down the crater.
Some Angel-type entity doing something involving fireing up his wings...
Looks a lot like the previous Angel in the tower, has a helmet with horns/wings, and apparently turns to fire/light. Tyrael has no such helmet normally.
Cain and Leah
We know Cain has spent the last 20 years trying to find out why there has not been a demonic onslaught after the Worldstone was destroyed, and either all that research has been done in Tristram Cathedral, or they just recently arrived there.
From Abd al-Hazir's entry about Gnarled Walkers, we know Wortham is close to Tristram, and at one point, Leah was there with her father. This could have been either before she met up with Cain or a brief trip to Wortham with her father during the research.
If nothing else, the two are both obviously surprised when the meteor comes rushing down, so it's an item that appeared unexpectedly.
You can find pictures in the Diablo 3 screenshot and picture gallery:
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System.  The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteor reaches the ground and survives impact, then it is called a meteorite. Many meteors appearing seconds or minutes apart are called a meteor shower. The root word meteor comes from the Greek meteōros, meaning "high in the air".