Tekkei

is a great bird Daiyōkai literally the size of a mountain who was mother of Princess Abi and queen of the demon birds. She was also part of a group of bird demons of the netherworld. Before Abi was introduced in the story, she ate the leader of a rival demon tribe in the area. That particular species turned out to be toxic and she was badly poisoned, forcing her to rest inside of her bird demons' mountain nest with only a hole in the mountain for her to see and communicate with the outside world. Her daughter Princess Abi used their bird armies to bring the blood of countless villagers for Tekkei to drink so as to dilute the poison. Later Naraku offered assistance in their quest to cure Tekkei, which they accepted but only because they intended to kill him themselves afterward. They had their suspicions and planned on betraying Naraku. When Tekkei finally healed she revealed her self to be a gigantic bird the size of a mountain and proceeded to attack Naraku along with Abi and swallowed him. Then Inuyasha's gang attacked them and they started to fight but suddenly Naraku burst out of Tekkei's head revealing to be alive and then killed both Tekkei and Abi and used Tekkei's blood to go to the netherworld.

Powers and Abilities
Immense Demonic Power: As the queen of the Demon Birds, Tekkei is a very powerful demon capable of easily defeating many smaller demons by devouring them alive.

Connection to the Netherworld: A unique power Tekkei possesses is the ability to enter into the demon netherworld as her demon bird subjects originate from it. It is this ability that Naraku came after her and her daughter, Abi, in order to obtain the last Shikon Jewel shard by killing her and having her blood spilled as a river that opens a portal there. This makes it an involuntary power as Naraku was able to use even after Tekkei's death.

Flight: As a bird Daiyōkai, Tekkei is capable of flying through the use of her wings though given her size, she is unable to do high-speed flight.

Pyrokinesis: Tekkei is able to breath fire in both the manga and the anime. In the anime, she also showed the ability to shoot fire balls from all over her body. Her fire manipulation is also seen to be tied to her very being as she is constantly wreathed in blue flames even after her death.

Immense Strength: Tekkei is a very large demon, giving her the brute strength of a creature her size where she easily demolished the mountain that she was residing inside just by moving out of it. She is able to move with ease and agility while flying and has shown that she is capable of creating whirlwinds and launching large boulders by flapping her wings.

Trivia

 * Despite being a Daiyōkai, Tekkei is not very bright. She had only just recovered from eating a yōkai she shouldn't have when she ate Naraku, yet another yōkai she shouldn't have eaten.

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