Bloodbending is a sub-skill within the art of waterbending, which allows a waterbender to hydrokinetically take hold and manipulate the various fluids within an organism's body. The technique is widely regarded to be the darkest and deadliest sub-skill of any of the bending arts, and the only known technique to put the user's mental faculties at risk. It was discovered and developed by Hama while she was detained in a prison specifically designed by the Fire Nation to hold waterbenders.
Bloodbending, as it was originally intended, was used by waterbenders to take control of other individuals and bid them to their will. Hama used the skill during the full moon when her abilities were peaked, immobilizing villagers who ventured through the forest at night and forcing them into a mountain prison.
In the years after her discovery, the technique's dark nature caused it to be banned from formal practice in the United Republic, though malefactors like Yakone still used the art to control the rotten underbelly of the nation's capital. Psychic bloodbending was developed during this time, otherwise known as the use of bending with minimal or limited movement, making its use imperceptible to those around the user.
In addition, the scope and practice of the art expanded, as the technique became accomplishable during daylight and other times when the full moon was absent. Bloodbenders became capable of subduing and incapacitating an entire courtroom of individuals simultaneously, and Amon even developed the ability of bending severing. In effect, this skill allowed him to disrupt a bender's chi paths and prevent them from bending, accomplishing the same effect as energybending in that it was longstanding and apparently irreversible.