A human's physical and astral body compared to a lesser demon's

The astral side (Japanese: 精神世界面アストラルサイド Kanji translation: Spirit World Side, Romaji: asutoraru saido; aka astral plane) is the realm of spiritual existence and the only surely known sub-side in the Slayers world.

Every living being has an astral body, which may be interpreted as one's soul. It is similar to an aura of sorts in appearance, and has a close connection to the owner's mind and consciousness; if the astral body is damaged, the mind becomes muddled. If it is destroyed, the creature dies instantly, although it may look unharmed physically. Evidence suggests that the destruction of one's astral body does not result in the complete annihilation of his personality; the essence of the person may leave to the afterlife as normal, at least in the case of humans.

Ragudo Mezegis's strike caused a rift to be torn in the spatial fabric, allowing a brief glimpse into the astral side, and severely damaging Xelloss

For most beings, the astral body is little more than a small but important appendage, but for mazoku, it is actually the creature itself. They actually live in the astral plane, and "project" bodies to the physical plane to be able to interact with it. Therefore, even a lesser demon's astral body is far greater than a normal human's; on the other hand, for the same reason it is also far more vulnerable. Whereas a human's astral body could be seen as a shadow of his or her physical body, a mazoku's physical body could be seen as a shadow of its astral body.

Objects and other nonliving matter do not have an astral body, although they can be given an astral signature by magical means, which allows them to be tracked from the astral side.

Nonmagical effects of the physical side do not extend into the astral side, and vice versa. Black, white, and holy magic can affect the astral side, but most shamanistic spells cannot. Astral shamanistic spells do work, as they specifically target astral bodies.