[quote=@Rai]It needs more details. Darkness in a soul is I assume you're equating to malice, or some kind of inherent impurity in their souls. Most forms of darkness is either legitimate shadow, lack of light, or a manifestation of shadow, An otherworldly property.[/quote]
What I was thinking would be a spell or power which would allow the Seraphim-Nayu to peer into the soul of their target, and then pull from it the darkest parts to manipulate the target into being unable to continue fighting.

The 'darkness' would vary between targets, but would be based on their most prominent personality trait, so a proud warrior could find themselves surrounded by people from their childhood or an old tutor mocking their abilities, while a brave soldier would be plunged into a flashback of an horrfiic battle where they lost someone they cared about. It wouldn't always be memories though, and some might find their deepest, darkest desires brought to life, tempting them into corruption, or their greatest fears brought forth to attack them.

This would be combining the Seraphim's manipulation of spiritual energies with the Nayu's knowledge of shadows and darkness. I was thinking this would be an 80% Seraphim/20% Nayu, who had been raised by Naru and trained by them. It would also combine elements of Divination, allowing for the scouring of the target's soul; Enchantment, to apply a persistent effect against the target; and either Illusion or Conjuration, depending on whether the thing which is drawn forth would be just a mental effect or an actual physical construct they'd have to face.

The spell or power would be something which could be cast at range with minimal effect, but I'd also be wanting my character to have a sword or spear made of spiritual energies which when used against a target would heighten the impact of the sorceries, making it easier to tear apart their minds from the inside. Mechanically I'd see this as just applying a boost to the effect of the spell, or undermining the target's defence against the effect after they were wounded by the weapon.