Do you remember the flowers of Tenebrae? You'll find they await you still, blooming from hill to vale.
Sylleblossom (ジールの花, Jiiru no hana?, lit. flower of zeal) is a flower native to the land of Tenebrae in Final Fantasy XV that grow in vast open fields. They are Oracle Lunafreya Nox Fleuret's favorite flowers. Sylleblossoms are vivid blue flowers that resemble gentianas. Gentiana is a divine Messenger who stays by Luna's side. The front cover of the Cosmogony book features the first Oracle holding a bouquet of sylleblossoms, highlighting the connection of the flower with the line of the Oracle.
When designing the flower, the developers picked the color before the shape. It was difficult to design because it would be a prominent element in the story and the designers wanted to make it disappear like dust. Elements of lavender, nigella, and iris were mixed to design the sylleblossom.
After sustaining a grievous injury, an eight-year-old Prince Noctis was sent to Tenebrae to recuperate where he befriended the young Princess Lunafreya. Luna showed Noctis a pressed sylleblossom in her red notebook and told him the flowers are used to weave the royal family's crowns. After the Niflheim Empire attacked Tenebrae, Noctis and Luna could not see each other again. The two keep in secret correspondence via the red notebook that is delivered between them by Luna's magical dog Umbra.
Twelve years later, Lunafreya is reported dead at the attack on Insomnia. The world mourns for her and across the land, sylleblossoms from Tenebrae are displayed in memorial. She resurfaces, however, and Noctis finally reunites with her, now his betrothed. After battling Leviathan, Noctis falls unconscious and is healed by the mortally wounded Luna. Noctis meets her in a dream vision on her field of sylleblossoms. At first, they are both children, but as the field begins to warp and Luna disappears into the blue abyss, she transforms into an adult. She sends out a sylleblossom to Noctis, revealing the Ring of the Lucii from inside its bloom, a royal heirloom she had been keeping for Noctis. When Noctis awakens days later, he has the ring, but learns Luna has died and that Luna's notebook has a message for him. Noctis opens the book to find another pressed sylleblossom.
On his journey to Gralea, Noctis makes a stop at Tenebrae. He finds the Nox Fleuret mansion aflame, and the grounds around their stop are blue with sylleblossoms. He learns Luna had met her brother Ravus on her field of sylleblossoms recently, where he had tried to persuade her to stop helping Noctis, not believing Noctis is the True King. A household servant mentions to Noctis how Luna would personally water the flowers, cultivating a wide, beautiful field of sylleblossoms that spreads across the lands of Tenebrae. When Noctis feels Luna's presence on the train to Gralea, a ghostly Luna appears to him amid glowing sylleblossom petals.
The sylleblossoms represent Luna and her connection to Noctis. The flower preserved in the notebook could represent Luna's spirit always watching over him. The falling petals appearing in the ending serves a similar role as the symbolism of falling cherry blossoms in Japanese culture. The young Luna told Noctis the flowers are used to weave Tenebrae's royal family's crowns. She, likewise, told him the Crystal's calling is to crown the True King. Luna giving Noctis a pressed sylleblossom, a flower supposedly for a royal crown, could thus allude to her role as the Oracle who helps the True King fulfill the prophecy.
In Final Fantasy XV: Episode Ignis, ghostly sylleblossom petals surround Lunafreya when her spirit departs. In Verse 2, blue petals swirl by Ignis who is healed of his injuries though he had offered his life as sacrifice to save Noctis, implying Lunafreya's spirit was what healed him.