You have been warned.
edit: Finished all acts, changed the title to something a bit more snappy.
A blue meteor falls from the sky. Blue! Magics! Let's investigate!
So the Player goes to have a look. Don't ask why exactly, he's the hero. It's what heroes do.
Townspeople/Guards/everybody: “Gosh, those are mad fighting skillz! I bet he's a nephalem!”
Leah: “Hi there, I'm Leah. Deckard's gone. But he's my uncle, so that makes me important. Search by the meteor, because he was two feet from its impact when I last saw him. I'm sure he's fine, though. Also, My mom is Adria and my dad is the Dark Wanderer. I'm totally not going to morph into Diablo when you last suspect it. Only, now you totally suspect it. Eh, don't matter, you will act totally surprised, kay?”
Player: “Whatever. So, Deckard is where the meteor is? How fortuitously coincidental! Two zombies with one explosion.”
Player finds Deckard Cain.
Player: “Hey Cain, what up with that meteor?”
Deckard: “I have no idea, and apparently no brain. Nah, just kidding, it's old age.”
Player: “So it's Tyrael, right?”
Deckard: “I have no idea what you are talking about.”
Player: “Go on, admit it, it's obvious. That blue is blue like those angel wingies. And it fell from the sky. You know, sky like: Heaven with a capital H.”
Deckard: “Why don't you get out of my hair and kill that Skeleton King?”
Deckard: “Because he makes old players happy, and adds more suspense before you find out what that mysterious meteor is about!”
Player: “I told you, it's Tyrael. Wanna bet?”
Deckard: “Leoric still drops phat loot.”
Player: “That's what I'm talking about!”
Player goes out on a MacGuffin hunt to kill Leoric, finds the impact of the meteor and Tyrael.
Player: “Sup Tyrael! Where's your wings?”
Tyrael: “I'm not Tyrael. I'm a mysterious stranger. I am so mysterious, in fact, that I conveniently have forgotten who I am!”
Player: “Dude, look, you're Tyrael. You're one of the Archangels or whatever, and you fell from the sky. Obviously, something is going on there...”
Tyrael: “I am a mysterious stranger. Who is this Tyr-Boy you keep jabbering on?”
Player: “Fine. Be that way. Let's get to Cain to sort you out.”
Deckard: “Look, a mysterious stranger who has forgotten who he is and can't remember! Quick, to the MacGuffin mobile!”
Leah: “He wants to say that you need to find the three pieces of his sword. Somehow that will help him remember who he is. Possibly by letting us perform a lobotomy on him. I just didn't say that, and I'm in no way going to be Diablo!”
Player: “... He's Tyrael...”
Plucky group: “Whatever, go find the MacGuffin, those stories are not easy to write!”
Player: “Aw man, who the crazy one here?“
Group: “There's loot!”
Player: “Okay, okay, I'm off. Besides, it's sweet to pulverize whole columns by spitting at them.”
Player finds the first MacGuffin.
Maghda: “Hohoho, I'm the exposition fairy, and since you have no clue where to find the next MacGuffin, I will conveniently tell you, even though I want them myself. But what's life without a little competition, eh?”
Player: “Are you the village idiot?”
Maghda: “Kill him!”
Player: “Whatever, see you at the next MacGuffin, exposition fairy.”