Where did Shiro get all those horrible power?

How did he get the power to control spirit, make plague and cast the Jade wind?

he is a very powerful individual, and it wouldn't be that hard to learn to control spirits if he lived in cantha, he was just very good at it. he made that plague as an envoy, when he had unlimited power. and the jade wind was the explosion of all the evil power leaving his body, alot of that power is from abbadon.

Shiro was one of the most powerful warriors of his time. In his Envoy form where he walked a corruption would begin or the "plague" as we know it, causing the horrific Afflicted. His job as an Envoy was to shepherd the dead on but he instead used these souls however he wished creating the Constructs.

This is my theory on where his power came from (power to create the Jade Wind). It cannot of come from Abaddon since Shiro did not have a clue about him at that time since he was being manipulated by Abaddons servant the Fortune Teller. When he slays the Emperor we see a strange power tranferring to him.

It could be that since the Emperor was only halfway through the ceremony when Shiro killed him, Shiro gained some sort of power since the ceremony was not yet over. This power could of been so great that when Shiro was at his last moment his cry caused the Jade Wind.

Yeah, that's the way I see it as well, since it's very clearly shown in the cinematic how something gets transferred from the emperor's body to Shiro. Power or a curse, you can take your guess on that.