What led Cain to suspect impending doom after the events of Diablo II: Lord of Destruction, but before Diablo III?

Question

I'm reading Diablo III: The Order right now, and the book begins post-D2, pre-D3, in The Borderlands as Cain is searching ruins for answers/clues to the coming darkness which would ultimately result in Diablo III. I've re-read the first chapter again, but I'm still unclear --- what exactly happened after Diablo II that led Cain to believe the fight wasn't over, and that he needed to search for clues about what might be coming?

There is are posts here and here which suggest the destruction of the Worldstone, which weakened men, implies that men would become stronger and perhaps start another war. But if I'm not mistaken, Cain is not worried about that, but rather a continuing/renewed threat specifically from the Burning Hells, not related to the Worldstone.

Answer 1

I haven't read the book, this answer is based only on the lore present in D2/D3.
Cain knows that (in D2) Andariel, Duriel, Mephisto, Diablo, and Baal were defeated. I believe that the Prime Evils' soulstones were shattered, but I'm not sure if the Lesser Evils have to have the same treatment for them to be 'defeated'. I say 'defeated' since we see their souls come back into the Black Soulstone in D3.

Regardless of the state of Andariel and Duriel, Cain knows that there are still two Lesser Evils out there. While we only saw the extent of Belial and Azmodan's power in D3, Cain likely knew their strength from ancient texts. I think Cain would continue to be vigilant, and watch for signs of Belial or Azmodan attacking Sanctuary.

Answer 2

I also have not read the book but going on the answer provided by @mmatthews, if Cain knew that Belial and Azmodan were still lurking about he had to suspect that they would eventually attempt another rise over man and attack Heaven once they were powerful enough as they had done in the past. Given his past experiences with the Prime and Lesser Evils none of it was going to be good for man. Thus he would need to find the evidence to back his claim so men were ready when that time came.