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It never appeared when his soul was damaged.
Nor it appeared when he took chunks of his soul to attack enemies.
Nor it happened when he was at the border of death.
It only appeared once to devour the Blood Progenitor's soul and force it to fuse with Noah.
And it appeared a second time now, to devour the seals that restrained Noah's soul.
Why? Because it hated to be restrained.
As a spawn of the chaos of ancient and mysterious origins, it was a being of unruly existence that desired to do as it pleased.
It had a strange personality that Noah had not yet understood.
But Noah had the faint thought that it might emerge when his soul was to be restrained or attack by a separate being capable of actually restraining or devouring his soul.
However, they were mere hypotheses, as Noah had prepared something else without even considering that the Spawn of Azathoth would help him.