Here's your corrected and expanded version with improved grammar, smoother flow, and slightly increased word count while preserving your original style:
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The instant the cultivator flew over the shadow, it let out angry and malicious roars that could not be heard by ordinary ears. The moment the cultivator crossed above it, the shadow, which had taken the form of a demonic-looking tree, suddenly extended its leafless, pitch-black branches toward the cultivator called Kaien.
Kaien, who was still trying to figure out how to escape from Grey, suddenly saw several slim branches emerge from beneath the pitch-black sea and shoot toward him like black spears.
Their speed was terrifyingly fast. Moreover, because the branches were the same color as the dark waters they emerged from, Kaien found it extremely difficult to notice them in time.
By the time he sensed the danger, it was already too late.
Pui!
Several shadow branches pierced straight through his body.
