Every time Sunny had inherited a part of Weaver's lineage, he had to endure harrowing agony. That was because his very being was assimilating something that was forbidden, and turning into something it was not meant to be.
The gods themselves had forbidden daemons from propagating, after all. So, by inheriting the legacy of Weaver, Sunny was going against the will of the gods — against the nature of his existence itself.
However, when a shadow cast by the Demon of Destiny fused with his own, there was no pain. There was no agony, either, and he did not lose consciousness, like he had when accepting Mind Weave.
However, the transformation he was undergoing was so profound and all-encompassing that it made Sunny feel instinctual fear. He was a creature of shadows, after all, so Shadow Weave was bound to alter the very foundation of him — the very depths of his vast, lightless soul.
And being changed so entirely could not be anything but frightening.
