Nereid Kyrie, also known as the Poem of Endless Green Sea, is a Sacred Gear currently wielded by Ingvild Leviathan, the Queen of Issei Hyoudou's Peerage. It is listed as one of the currently known 5 New Longinus, being a high-tier Longinus.
One of the newly manifested Longinus, it awakened inside of Ingvild Leviathan, a half-human descendant of the Leviathan clan. Due to Ingvild being in a coma because of her sleeping disease, the manifestation of her Logninus awakened her from her sleep. She was later taken to the Grigori who helped her in analyzing her power, concluding that it has the ability to affect Dragons and the sea. During that time, she was ambushed by the Greek Goddess of the Night — Nyx, who implanted a mind control spell inside of Ingvild as she hoped to harness the power of Ingvild's Longinus to use against the Two Heavenly Dragons. Ingvild was forced to use her Longinus against Issei Hyoudou as she rendered him powerless with her song. During Issei's fight with Nyx, he asked Ingvild to sing for him and use the Longinus' ability in reverse, in order to strengthen him and thus giving Issei the power to defeat Nyx.
Currently, Ingvild is still training to control her Longinus as she cannot use it to specifically target Dragons to weaken with her ability, while also learning how to strengthen them to an enormous degree.  Her ability to control the sea is still immature compared to her ability to control Dragons.
While Nereid Kyrie doesn't have a physical form, when using it, Ingvild's eyes glow in a lavender color as the area around her is enveloped in lavender light particles.
Nereid Kyrie grants the user the abilities to control and strengthen Dragons, as well as the Sea.
It can be used to enslave Dragons to the possessor's will as well as making them powerless or strengthening them. It also grants the user instinctive knowledge of Dragons and can be used to put them to sleep. It is shown to be powerful enough to dispel the armor of Issei Hyoudou who was wearing his Crimson Armor, with Ddraig stating that it could potentially even affect Issei in his Diabolos Dragon God armor. Ajuka Beelzebub stated that the power of this Longinus could potentially control the Dragon Gods such as Great Red and Ophis.
It is stated that the power to control the sea could potentially allow the user to control the entire ocean and sink a urban area and an entire country. According to Rias Gremory, the Longinus could potentially be turned into a Sub-Species which can also control lakes.
As a High-tier Longinus, it has the potential to destroy the world, through an army of empowered dragons and control over the seas.
- Paipod (乳音拡声, Paipoddo): Also known as Breast Song Reinforcement. By using Issei's Diabolos Dragon God as a conduit, Ingvild can empower Issei and his allies who are present. The song can be heard through the breasts of women as the area is bathed in lavender light particles.
- In Greek mythology, the Nereids, are sea nymphs (female spirits of sea waters), the 50 daughters of Nereus and Doris, and sisters to Nerites. Also they often accompany the God of the Sea, Poseidon, and can be friendly and helpful to sailors.
- Kyrie, is a transliteration of the Greek name, Κύριε, vocative case of Κύριος (Kyrios), and is a common name of an important prayer of Christian liturgy, also called the Kyrie eleison (Ancient Greek: Κύριε, ἐλέησον, lit. 'Lord, have mercy').
- Nereid Kyrie is one of two Longinus (alongside the Divine Dividing) to be wielded by a descendant of the original Satans.
- Both Sacred Gears are also dragon-related.