First Heavenly Civil War

Lucifer's Rebellion was a conflict fought in Heaven, a long time before the creation of the Universe (13.8 billion years ago). It resulted in the victory of God and Michael, the greatest consequence being Lucifer's exile out of Heaven and, far later on, his imprisonment in the Cage.

History
God created the first and strongest celestial beings, the archangels. Michael was created first, being the mightiest, but also most obedient. Lucifer was created second, being the most beautiful and also, the most prideful. Raphael was created third, being cold and not displaying much emotion. Gabriel was created last, having a childish personality and also, having a great sympathy towards the human race (therefore, Gabriel is the only one that truly became what God wanted - human loving).

In order to create the infinite Universe, God had to defeat His sister, the Darkness. The combined strength of Him and His archangels was enough to imprison the Darkness into the Mark. God passed the Mark to his most trusted lieutenant, Lucifer. The Mark, however, started corrupting Lucifer.

Ages later, after having created the Universe and the human race, God asked all his angels to bow down to his new creations. Lucifer refused, stating that they're flawed, that they're an abomination. He, then, declared war to God and tried to rebel against him. Raphael sided with God, and Gabriel left Heaven, as he could not see his family break apart. Lucifer asked Michael to side with him, however Michael refused, and at the command of God, defeated Lucifer and one third of the angelic host, thus crushing the rebellion.

Lucifer was the condemned to Hell. In order to taunt God, the twisted Lilith, a human, into becoming a demon. Enraged at this, He imprisoned Lucifer into the Cage. He would later be released by Sam, after killing Lilith, thus breaking the 66th seal holding Lucifer in his Cage.

After being imprisoned a second time, he would later be released another time, which might be permanent, possessing Castiel, in order to anchor himself to the Universe.

Casualties and Consequences
The fate of one third of angels that were banished along with Lucifer is unknown. They might have become demons, or simply died. There are no other casualties that are known, though the Rebellion might have killed other angels that sided with God, during the battle in Heaven.

The biggest consequence was the linkage of the Universe and Hell, evil being able to exist within Earth's plane of existence. The twisting of Lilith and creation of the first demon let to the creation of the Knights of Hell and the rapid increase of the numbers of demons, existing in Hell and on Earth.