Windmill of Time

The is a mysterious contraption that appears in Hanyō no Yashahime.

History
When Kirinmaru visits Sesshōmaru's dog-demon mother, he was curious about the windmill in the sky right in front of her palace. She has mentioned that she hasn't seen Akuru in such a long time, including that of Akuru's pinwheel.

After Zero have been struck by Setsuna's dormant Blood Blade, he observed from his ship that the Windmill of Time had finally began to turn if gradually.

At Mount Musubi, Kirinmaru's daughter Rion tells the Half-Demon Princesses Towa, Setsuna, and Moroha that he's after Akuru and their pinwheel to move the Windmill of Time that will cause the world to fall into the "Degenerate Age", the very same thing the Tree of Ages warned them about.

As Kirinmaru stands in front of the Windmill of Time, he spots Akuru in attempt to catch him. After Zero dies as seen in flowers from the ground of illusion around Nanahoshi's mansion, the Windmill of Time had finally began to turn if gradually again.

During Kirinmaru and Moroha's fight, Moroha has a flashback as what Rion warns them about Akuru and the Windmill of Time back at Mount Musubi.

As Takechiyo takes Towa, Setsuna, Moroha, Jaken, and Akuru to the manor of Towa and Setsuna's grandmother where they'll meet her, the Half-Demon Princesses saw the Windmill of Time. Due to the presence of Meidomaru, it remains immobile and ultimately catches fire.

Trivia

 * The Windmill of Time was first teased in BURN.
 * It's inspired by the Space-Time Door that Sailor Pluto, Sailor Guardian of Space-Time, guards from Sailor Moon manga, and 1992-1997 and the ongoing 2014 anime.