The Saigyou Ayakashi is a youkai cherry blossom tree, and is the largest cherry blossom in the gardens of Hakugyokurou. It has existed in the Netherworld longer than the Ghost Princess, Yuyuko Saigyouji, herself. Some of the spirits in the Netherworld remember the Saigyou Ayakashi having the most beautiful blossoms they had ever seen, but the cherry tree has not bloomed for ages. There currently seems to be a powerful seal within the Saigyou Ayakashi which drains most of its energy away, leaving it unable to blossom at all during spring. Whether the tree is sealing something, or whether the tree itself is being sealed, remains a mystery to most outsiders.
A long time ago, there lived a divinely talented poet who traveled around Gensokyo. When the poet became aware that the hour of his death drew near, he chose to begin his eternal rest under the most beautiful cherry tree he could find. After the poet's death, word of the beauty of the cherry tree began to spread across the land. That cherry tree has grown even more beautiful since then, charming many people into resting in peace beneath its blossoms. The tree with the mortuary charm became known as Saigyou Ayakashi, and developed the powers of a youkai after a while.
As a youkai, the Saigyou Ayakashi's beauty and charm increased even farther, driving everyone who approached it insane with deadly peacefulness. The cherry tree had drained too many human souls and had become a danger to everyone. Eventually, the beautiful blossoms of the Saigyou Ayakashi were sealed away by some unknown heroes, using Yuyuko's human soul as the main component. The seal would freeze both the blossoms and the soul in place together for eternity.
During the events of Perfect Cherry Blossom, Yuyuko Saigyouji, the ghost of the human Yuyuko, attempted to overpower the seal on the Saigyou Ayakashi in order to both see the legendary blossoms, and to resurrect the soul that was sealed within. However, Yuyuko was unaware that the sealed soul was in fact her own, and thus all of Yuyuko's attempts to unseal the soul failed due to her unconscious self-preservation.
- The poet referred in the Saigyou Ayakashi's story is likely Saigyou, an ancient Buddhist poet who often sung of his love of cherry blossoms and his desire to die beneath flowers.
- Ayakashi is a general term for supernatural beings, very similar to youkai. The tree's name would mean something like "Youkai (Tree) of Saigyou".