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 Sacred Tree of Ages had 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, that many people to the place with the intent to travel through time, but one after another, those who have not been chosen have been sucked into space-time and perished. After Setsuna severed his with the Windmill, he goes to the, and since Akuru gave Towa the pinwheel, she blow it to open the Doors of Time, as Akuru and the Half-Demon Princesses pass through except for Takechiyo and Jaken as Takechiyo took them to the Doors of Time, Kirinmaru with Rion will get there since he was furious until he tried to use the Bakuseiken to slash it to stop the Half-Demon Princesses from going back to the  but was too late, immobile, and ultimately catches fire as he use his right arm to call Osamu Kirin to let him to the Reiwa era and made it pass through, before it closes comes it remains marks the Degenerate Age is about to begin as lots of demons coming out. As the demons come, Kirinmaru use the blast of shockwave to get rid of demons so they'll never interfere. Due the infinitely powerful, destructive Grim Comet appearing in the feudal era, it fades from existence.

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.