Although Slann worlds can be found throughout the galaxy, their society remains closed to other races. Some worlds are highly advanced while others have fallen back to a primitive state. Wahtever their their technological advacement, they rarely ever trade with outsiders and even prove to be unpredictable due to the fact that there is little that the Slann need or want. As such, any traders encountering a tribe of Slann feel as if they were being watched or studied, with no interest in any exchange of goods.
They hold no equals in the realm of psychic-philosophy or mystic-technology as they fulfill themselves by the study of spiritual lifeforces along with the secret powers of other realities, believing that the object of life is to perfect the mind in order to become an immortal spirit.
They are bleived to be either degenerate sbjects or descendants of the Old Ones.
Source: Warhgammer universe