Q'yth-az, the Crystalloid Intellect, is a Great Old One that is depicted as a towering mass of crystals, residing on the lightless planet Mthura.

While appearing as a mass of crystals, it is obviously sentient. Although, unlike most of the Old Ones, it is incapable of movement and only ever positioned in one place. However, it can spread about its crystalline form across various regions of the planet, thereby traversing through infection.

Should one come in contact with Q'yth-az, it is said that the crystals will spread about their being, replacing every organ, bone, and flesh with crystalline substance thus not only effectively killing their consciousness, but ultimately becoming a servitor of Q'yth-az.