Bloodbending

Bloodbending is a specialized technique within the art of waterbending that allows a waterbender to hydrokinetically take hold then manipulate the various fluids within an organism's body. The technique is referred to as the darkest and deadliest sub-skill of any of the bending arts, and the only known art to put the user's mental state at risk. The only Bloodbender was Hama, and became extinct after her death.

History
The technique was discovered by a young waterbender named Hama while she was detained in a prison designed by the Fire Nation specifically to hold waterbenders. During her detainment, she concluded that all life contains water, and proceeded to develop the knowledge and skills of the art by practicing on elephant rats. The forms and styles used by bloodbenders resemble how a puppeteer controls a marionette. Whereas waterbending uses flowing motions with the arms, bloodbending seems to require a more rigid and abrupt form of movement, further highlighting how different it is from the primary waterbending forms and its other sub-skill, healing. Due to its extreme nature, only a handful of waterbenders have demonstrated the ability to bloodbend. The technique is deemed by most waterbenders to be a dark art and is rarely used in combat. The complexity and sophistication required to perform the art usually only allows for its use during a full moon, when a waterbender's power is at its absolute peak. Some bloodbenders have learned to use the art without the need of a full moon.

Limitations
Despite being one of the most powerful and dangerous techniques in bending, bloodbending is limited in the sense that it is usually only performable under the full moon, when a waterbender's abilities are peaked. This is due to the fact that bloodbending requires far more sophistication and precision than conventional waterbending, thus limiting its use to only when a waterbender's capabilities are culminated. Waterbenders who expend a significant amount of time and effort in training, are able to use the technique without the assistance of the full moon.

The degree in which a bloodbender can manipulate another person's body only occurs on a physical level, leaving the victim's mental faculties intact. Consequently, a bloodbender is unable to use the bending abilities of the victims, as bending also relies on the mental state of the bender. They can, however, restrict movement to stop an act of bending, as well as reposition attacks in progress by discretely manipulating the user. Skilled bloodbenders are able to levitate people off the ground, as well as knock several individuals unconscious simultaneously.

Lastly, bloodbending seems useless when applied to a fellow waterbender of the same caliber, if not superior, to the user. Bloodbending is also nullified when the Avatar channels immense amounts of cosmic energy through the Avatar State, and the Avatar can break through a bloodbenders hold instantaneously.