The shadow spikes caused heavy injuries on the deer but it was not enough to totally kill it. This is the Magic Beasts' edge over humans.
Human spells need to be powerful enough to kill magic beasts. As for the beasts, one solid hit on the human bodies might already be enough to kill.
Calter poured more mana on the sword in his hand and the black aura around it intensifies. Since the deer is still controlling the surrounding trees, it would be harder to get close to it
Calter poured strength on his arm while his eyes locked on the deer.
Swoosh!
The sword traveled in a straight line and penetrated the deer's face, finally killing it as a result
Nature calmed down and Calter didn't immediately approach the deer to take the crystal diamond. He decided to survey his surroundings first for any unexpected variables before cautiously walking towards the corpse of the deer