Like other Longinus, Nereid Kyrie grants the user two distinct abilities:
1. one is the power to manipulate Dragons by using their singing voice and;
2. manipulating the sea.
In singing alone, the user could enslave Dragons, making a Dragon fall asleep, making them powerless, which was first shown as Issei couldn't continue wearing his Cardinal Crimson Promotion, affecting Ddraig, who said that it might even affect Diabolos Dragon God. Ajuka Beelzebub stated that even Dragon Gods such as Great Red and Ophis could be potentially controlled.
An additional trait, is that the user can detect Dragons, being able to tell that Issei had Ddraig sealed within him via Boosted Gear. Aside from being able to weaken Dragons, Nereid Kyrie can be used to strengthen them too, and through using Issei in Diabolos Dragon God as a medium, it can be used to strengthen others.
It's second ability of manipulating the sea could be used to control an entire ocean, letting one sink an urban area, reaching the point of an entire country. According to Rias, it could develop into a Sub-Species, which can control lakes too, because of Ingvild Leviathan's inherit Demonic Power, Sea Serpent of the End.
As a High-tier Longinus, Nereid Kyrie has the potential to destroy the world, through using an army of controlled Dragons and manipulating the sea.