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Blake had seen half-demons in his previous life. Despite how everybody has surprised by Erdrich transformation, Blake knew very well of these types of transformations. They were most of the time, unique to pure half-demons. Because the power of a demon was so strong it conflicted with the other human half, the demon half would attain the power to enhance the human half, creating a series of different transformations often not even present in low-ranked demons. This was a unique capability of half-demons, and as they grew stronger, such a power developed even further. Erdrich was a talented boy, Blake had realized this long ago since he first confronted him, he was bound for greatness.
Check "The Protagnist's Sister Is Actually the Strongest"!