Kuro

WARNING! The following text contains spoilers!

Kuro is a large, black owl that snatched Sein from the Spirit Tree, thus removing the light from Nibel. She is the main antagonist of Ori and the Blind Forest.

History
On the night of the Light Ceremony, Kuro was feeding her three chicks, and left to gather more food for them. While she was away from the nest, the Light Ceremony started, and the Spirit Tree emitted a massive explosion of light. Kuro's chicks, being weak to the light, crouched in fear. Kuro immediately flew back to the nest to protect her children, sensing that they were in danger. However, when she arrived back at the nest, it was too late. Her children were already dead. Enraged by the loss of her children, and resolving to protect her last egg, Kuro's eyes turned from orange to white, and she flew to the Spirit Tree to unleash her rage upon the Spirits, and steal the tree's light. From that point, the land of Nibel became a dark, and nearly uninhabitable forest.

Pursuing Ori
After restoring the Element of Water and escaping the flood of the Ginso Tree, Ori found itself at the top of the forest, and awakened the sleeping Kuro. As Ori fled, and dove from the top of the forest, Kuro made an attempt to kill it. However, thanks to Gumo, Ori was saved. From that point, Kuro continued her pursuit. Upon entering the Valley of the Wind, Ori found Kuro guarding the passage to the Misty Woods. Sein saw a loose rock above, which Ori released to hit Kuro on the head, making her fly away. Later, after restoring the Element of Wind to Nibel, Ori discovered Kuro's last remaining egg, still unhatched. Kuro immediately arrived at the nest, and began pursuing Ori once again, but Ori manages to escape. Finally, after restoring the Element of Warmth, Kuro arrives on the scene and chases Ori once again.

Sacrifice
Ori manages to escape Kuro once again, but just barely, and is knocked out from injury. Ori wakes up and makes an attempt to reach the Spirit Tree to restore Nibel's light. Kuro then arrives, grabbing Ori with her Talon and tossing it to the ground, knocking it unconscious. A resurrected Naru arrives to the scene, and goes to hold Ori in her arms. Seeing Naru's love for Ori reminds Kuro of her own love for her children, and she looks to her nest to see the fires of Mount Horu creeping toward her unhatched egg. Realizing the grief she has caused, and knowing that she can not reach her egg in time to save it, Kuro takes Sein back to the Spirit Tree. An explosion of light kills Kuro, and the forest of Nibel is restored to its original form. Kuro's last surviving egg is found in Naru's cave, and is seen hatching in the end.

Trivia

 * Kuro is the Japanese word for black, which suits Kuro well, for being a black owl.