The Mask of Shadows gives War the ability to glimpse into a parallel dimension, allowing him to see things that would otherwise be unnoticed or innaccessible. The mask appears on War's face when equipped and the eyes will glow orange when used. this gives the world an orangish tint. At some points in the game you will find an object in the real world which is completely transparent but is outlined in black, upon entering the Shadow Realm, these objects reveal themselves and can now be interacted with. Examples of objects only usable with the Mask of Shadows are chests, shadowflight boosters, hookshot points, or anvils with the Armageddon Blade pieces. Typically, to get to the area with an Armageddon Blade shard, you have to use the Mask of Shadows to see the pieces of the environment needed.
The mask is obtained from Azrael when you reach Eden and is used the find the pieces of the Armageddon Blade so it can be reforged.
The Tormented Gates can give War the ability to see into the Shadow Realm before he gets the mask.