Soul Eater

Soul Eaters are a ghost-type monster that was first seen as the boss of the Third Wave. They are also native to Glass' World as Soul Devourers.

Abilities
Soul Eaters as ghost-type monsters. As such, they are immune to physical damage and must be attacked with magic. They can attack the souls of other beings directly.

When a soul is eaten by a Soul Eater, it cannot be reincarnated or revived. They can sometimes be captured and used expressly for that purpose.

Inter-dimensional Soul Eater
The third wave's final boss was able to use advanced magic compared to the regular Soul Eaters. It was able to gather powerful dark magic in its mouth and shoot the explosive energy.

Appearance
Soul Eaters look like ghostly fish, draped in white cloth. They have a dark, evil-looking face with red eyes and long fangs. They do not have physical bodies.

Volume 3
Soul Eaters were first seen during the third wave. They hid as shadows, controlling and reviving monsters like the skeleton captain. They were able to trick Itsuki, Ren, and Motoyasu into fighting an endless battle until Naofumi showed up. Raphtalia used her light magic to reveal the Soul Eaters as the shadows. Once the swarm of Soul Eaters was revealed, they merged into the wave's final boss - the Inter-dimensional Soul Eater.

Naofumi and the other heroes tried to attack the boss Soul Eater but hardly caused any damage. Naofumi transformed his shield into the new Shield of Wrath. Using the Iron Maiden ability, Naofumi was able to defeat the Inter-dimensional Soul Eater. After Glass confronted the four heroes, she defeated an Inter-dimensional Soul Eater in one shot. After their confrontation, Naofumi used the remains of the Soul Eater boss to unlock a new shield.

Volume 9
During Naofumi's final confrontation with Kyo, he was concerned that Kyo would use his technology to revive himself. In preparation, they acquired an ofuda to summon Soul Devourers - Soul Eaters of Glass' world. After they killed Kyo's body, they activated the ofuda. Raphtalia then defeated his ghost and then the Soul Devourers ate the remains of the ghost.

Apparently, Kyo had used Soul Devourers to destroy Trash #2's soul.

Volume 16
While Soul Eaters are not native to Raphtalia's world, a similar monster called a Soul Vacuumer is. This monster is a cousin of the Dune monsters and was used to eat the souls of Takt's harem during their execution.