WARNING: This article contains extreme SPOILERS. Stop reading now if you don't want to know how Diablo 3 ends.
The cutscene opens on a heavenly caste where a powerful archangle, Imperious, prepares to battle Diablo, who has taken the form of a little girl.
My enemy, says the archangle. You cannot hide from me no matter what form you choose to wear. Let your true self be revealed, Diablo!
A fiery blaze transforms the girl in a demonic monster, and the two engage in battle. Imperious fights bravely but is overwhlemed by Diablo. The scene then cuts to an image of a drawn cloack around an invisible body. A human narrator speak:
Discord ehcoed throughout the high heavens. Once a place of supreme harmony. Diablo had overtaken completely and rampaged throughout the realm of angels, turning its proud and nobel inhabitants to shadows of what they once were. Everything was lost and I surrendered to despair. Yet once again,the heroes, mortal heroes, never wavered. The champions rescued the archangels or hope and fate, inspiring me to continue the fight against Diablo. At the great crystal arch, the source of angelic power and the birthplace of our divine virtues, a mortal saved us all, and struck down the prime evil.
The scene then cuts back to the arch, where we see Diablo fall from the top, disintegrating as he plummets down to the ground. An angel narrates the end of the story:
In the end it was a mortal heart that saved two world's from ruin and cast down the prime evil, forever. A new day breaks for both angels and men. For mankind's greatest challenge rose to confront the darkness that we in a prime would not face. My brethren, I will take my place among you once again, but this time as a mortal. Since justice has been met this day I will now stand as wisdom. On behalf of those who risked all to save us. Forever more, we shall stand together. Angels and men in the light of this glorious new dawn.
Diablo 3 ends on an uplifting note, wrapping up a trilogy of RPG's in which evil often appeared to persevere over the forces of good. According to YouTube commenters, the final gameplay features an epic battle between the player and Diablo in a fight to the death.
A number of commenters on YouTube also noted, with a degree of criticism, that Diablo's character in the third game is portrayed as somewhat feminine or androgynous. They argue that this takes away from the terror factor that made Diablo 1 and 2 so scary and so much fun to play.
You can watch a collection of all the Diablo 3' cut scenes here.