Darksiders

After defeat Straga and watching the cut scene, you will be in Eden.
Collect the health and wrath chests if needed, then walk forward, another cutscene will follow and you will receive the Mask of Shadows
Story Progression Achievement

After you defeat Straga, you will enter the "Garden of Eden" with the Angel of Death.

You will be rewarded with the Mask of Shadows in this location. But, you have to fight "Shadow War" here. He will do the exact moves that you do and have the same weapons that you do.

The method that I used was to lightly attack him and then jump/dash out of the way. After a while, he will turn into Chaos Form. I suggest that you turn into Chaos Form as well. In Chaos Form, just keep hitting him. It doesn't matter if you take much damage because he will die before you if you keep hitting him.