Were said the lore?

Tyrael alone bested Azmodan ?

Azmodan created the demonic hellflyers after he nearly lost to Tyrael in battle thousands of years ago. The angels made glorious use of their great luminescent wings to outmaneuver the demon host and assail its commander. Frustrated by his near defeat, Azmodan began breeding winged demons capable of tearing the angels asunder

As far as I know, it didn’t explicitly say that Tyrael bested Azmodan alone. We also don’t know, if the battle where Tyrael nearly slew Azmodan was the same as the battle where Azmodan nearly lost due to the angel army making use of their wings.

One thing for certain, is that if Tyrael wasn’t alone when he bested Azmodan, then it’s safe to assume that Azmodan wasn’t alone either.

Will their be a book for diablo 4? Like the book of Cain which was amazing! Knowing more indepth about the lore is fascinating.

Most likely. I’m sure there will be more external additions for the lore as we approach Diablo 4’s release date (which won’t be soon).

Just hope it wont take as long as D3 took…