Now then, there are two kinds of shadow. The corruptive, and the destructive.
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The corruptive is a strange thick substance that covers the nearest living being (usually its creator) and takes control of their mind. One example of this is Wilfre. He created shadows, then tried to destroy the world. These shadows can be overcome with enough willpower. However, they can be extremely dangerous. Everyone has some in them, but it only thrives if they focus and develop on it.
These shadows are the substance that make up the Dark Master, all evil shadows, part of Shaden, some of Shadowbat's altered form, and more. It is unknown what this is made of or caused by, but it is sentient.