In PoE1 Trialmaster blamed Doryani for destruction of Vaal. And he is right,

In PoE1 Trialmaster blamed Doryani for destruction of Vaal. And now after Fate of Vaal it's actually makes sense.

Before Fate of Vaal it seemed that Cataclysm that wiped out civilization of Vaal is purely Atziri's fault. She communed with a Beast and things go nuclear.
But now...

1)Doryani invents time-travel gates. His inventions for some reason integrated into Atziri's Temple (most likely technology stolen by the Architect like he stole design of Doryani's Triumph crab-mecha).
2)Doryani reactivates time-travel gates of Atziri's temple in present time with intention to persuade Atziri not to proceed with her Beast plans.
3)Present time Exile arrives with Doryani at the right moment and witness "commune with a Beast" go smoothly! No nuclear blast! But then Exile beats Atziri's so hard that she interacts with a beast two more times and only after that things go into overload and then Nuclear.

So Doryani is the one to blame for the fall of Vaal. His invention and his interference with a "commune with a Beast" process destroyed his civilization. Trialmaster is right - Doryani is one to blame. Also Exile played a part in all this.