In Mai-HiME, the HiME Star is an astronomical object. It is depicted as a glowing red object in the sky, located to the lower-right of the moon and is the origin of the HiMEs' powers. As the series progresses, the Star is seen to grow larger as it approaches the earth. It is later discovered that the star is surrounded by a rock-like structure, akin in shape to the sigil of the HiMEs. In the anime, only the HiMEs and the First District can see the Star, whereas it is visible to everyone in the manga.
The Obsidian Lord states in the final episode of the anime that the Star is also the source of his powers. When it is destroyed, he is sufficiently weakened to allow the HiMEs to destroy him and release his vessel, Reito Minagi. Toward the end of the manga, it becomes his Obsidian Palace.