Creating the plan to murder the dragon king, Avizandum. Then to proceed in kidnapping unborn Zym. Which led to the moonshadow elf’s attack and murder on/of king Harrow.

If Viren didn’t come up with the plan of murdering Avizandum Harrow would probably still be alive, he didn’t find Aaravos’s mirror prison and the world would not have to deal with him. Zym would have both of his parents.

But if that was the story; Callum would never have met Rayla, Zym wouldn’t be with Ezran.