Prime Souls are souls that have grown so powerful that they no longer require a Husk to manifest a physical form; they simply exist out of sheer willpower.

Such souls are noted to be incredibly dangerous, as well as incredibly rare. Prime Souls are dangerous enough that the Council of Heaven attempts to prevent their occurrence at any cost, seeing fit to imprison souls they suspect of having the potential to become one.

(This is a species from the game ULTRAKILL)