In the black soulstone cinematic, Azmodan says "one by one *OUR* brethren fell right into your trap.. But I defy you." This pretty much proves he's speaking to Diablo.
Here's my personal take on the plot [potential spoilers..?]:
-Diablo is unleashed in Diablo 1, is defeated but compels the champion to put his soulstone in them.
-He charms adria to have sexy time with him so he can impregnate her, thus leaving an unsuspecting vessel of his spirit in sanctuary (adria was a powerful witch, maybe because of this she was closer to being the almighty nephalem than the average human)
-Diablo travels to the east to free his brothers: Not to really free them, just to manipulate them and then have them truly destroyed (sent to the Black Abyss).
-Once they are freed, he convinces Baal to travel to the worldstone chamber.
-Once Mephisto is destroyed, Diablo allows himself to be defeated so he can later manifest himself in the semi-nephalem/demon Leah.
-Baal is defeated at the worldstone chamber, but contaminates the worldstone, forcing tyreal to destroy it. All soulstones are destroyed, sending them to the Abyss.
-Now that the worldstone is no more, angels and demons are free to enter sanctuary. This does 2 things: 1) turns sanctuary into a battlefield, distracting heaven and hell from his efforts to become the prime evil, and 2) provides opportunity to have Azmodan and Beliel enter sanctuary to be destroyed and sent to the Black Abyss.
-Diablo's dormant evil within Leah compels her to locate the black soulstone, which has the power to bring demons back from the Black Abyss. He has her use it to summon him into her body, becoming more powerful than ever since it's Leah, the last remaining Horadrim and semi-demon/nephalem. He then uses it to summon the souls of his brothers into his new body, under his control (hence the 3 heads on diablo in cinematics). His ultimate goal is to absorb all of the 7 evils to become THE prime evil, Tathemet, and destroy heaven once and for all!
I have not played Tyreal into this plot either, because I have a few theories as to his significance. Shazaam!
Edited by deadpenguins#1190 on 5/8/2012 1:38 PM PDT