Antimatter Tentacles

The antimatter tentacles, or simply tentacles are the product of Yanagisawa's antimatter research and the material Korosensei is composed of.

The tentacles are a major plot device used in many significant events, such as Korosensei's creation, Itona Horibe's character arc and the revenge of Akari Yukimura.

Overview
The tentacles are the by-product of Yanagisawa's attempt to produce antimatter in the human body. Although flexible and soft, the tentacles can possess large amounts of strength and speed when weaponized. Since Korosensei is vulnerable to Anti-Sensei Weapons, the tentacles are also weak to them and will disintegrate upon contact.

Upon contact with water, a tentacle will absorb it like a sponge, and its mobility will be hindered. This can be partially negated if mucus is used to turn it into a gel; if the tentacle's mucus were to be depleted, this protection is unavailable.

According to most individuals who have been implanted or augmented with the tentacles, the latter will ask them what they wish to be and will act (in a way) accordingly. Notable examples of this occurrence include the following:
 * Korosensei requested to be weak, which prompted the tentacles to riddle him with numerous weaknesses as well as give him his current appearance.
 * Itona Horibe desired strength, so the tentacles forced him to focus on fighting and winning.
 * Akari Yukimura wished to become an assassin who blended in until the ideal opportunity presented itself. In response, the tentacles concealed themselves within her neck until her first attempt to assassinate Korosensei.
 * Reaper wished to be noticed by Korosensei and was made stronger and more powerful than him.

Tentacle Creatures

 * Main articles: Korosensei § Powers and Abilities, The Reaper § Powers and Abilities

The first instance of the tentacles. These life forms possess numerous powers and abilities, such as being able to regenerate upon damage and tremendous speed and strength.

However, tentacle creatures carry a risk of explosion once its cells reach the end of their division cycle. This depended on the size of the organism; the smaller it is, the higher the risk of explosion. For Korosensei, as he is both quite large and the progenitor for other tentacle creatures, his risk of explosion was less than one percent at best. It is even possible to remove this risk entirely, as shown with Reaper II's tentacled form.

Human Body Tentacle Weapon
A human can wield the power of the tentacles by injecting the tentacle seed into their bodies. Upon being implanted, the tentacles will bond themselves to the user's neurons and a varying number of them will sprout from the place of injection. Once this short process is complete, the tentacles will act on the will of the host.

In turn, the user will experience a multitude of harmful side effects and symptoms. As the tentacles and the human body have vast genetic differences, the most common symptom is excruciating pain. As stated by Itona and Kayano, the tentacles will absorb most of the host's energy through their neurons and as a result, will significantly hinder their natural abilities.

Without regular maintenance, the tentacles will quickly eat away at the user's mental stability and life force until the user dies. This occurrence can be identified through their eye color; the user's irises will turn black with yellow pupils and their scleroses will turn red. A notable example of this phenomenon is when Kayano had endured the pain of the tentacles for nearly the entire year without any upkeep, and became deranged in a few dozen seconds upon using her tentacles to their full extent.

Side Effects
Yanagisawa's research team compiled a full list of side effects for this variant of the tentacles: "#Acute Pain: Combining the human body and a genetically different life-form causes extreme organ rejection. The transplant recipient will experience pain on a daily basis. It requires a massive amount of energy to stabilize the recipient and prevent rejection.
 * 1) Decline in Performance: The bonding with the recipients neurons initiates a decline in the precision of neurotransmissions, resulting in a 10% to 30% decline in mental and physical performance. Therefore, the recipient must either receive further body modification or use their tentacles to compensate for the decline in physical performance if they are to be used for battles on warfare.
 * 2) Mental Instability: Because the tentacles are linked directly to the recipient's neurons, in some cases, excess tentacle energy will overstimulate the brain. This overstimulation can trigger bouts of rage which can physically damage the recipient and are extremely dangerous to both the recipient and others.
 * 3) Metabolic Instability: The recipient's body temperature is frequently observed to rise while the recipient simultaneously experiences a chill in the thoracic region. This is probably attributable to the fact that the organ, which is equivalent to the heart of the Perfect Tentacle Creature, did not originate inside the body of the transplant recipient. The chill is merely a sensory abnormality, but the rise in temperature may have negative ramifications for the body and mind of the recipient if it continues for a long time.
 * 4) Aversion to Water: Due to osmotic pressure, the movement of the tentacle is dulled when it comes in contact with water; thus recipients tend to instinctively avoid it. However, water merely destabilizes the balance of the tentacles, and no serious repercussions are currently known to occur.
 * 5) Change in Eating Habits: There is a tendency for recipients to develop a sweet tooth. This is thought to result from the recipient requiring a large amount of energy to maintain a connection with the tentacles; however, no serious repercussions are currently known to occur from this condition either.
 * 6) Major Obsession with Certain Body Parts: The recipient will develop a strong obsession with soft-tissue parts of the human body. It is presumed that the tentacle feels a strong sense of fellowship with these body parts because of their similar consistency. However, this preoccupation could be linked to number 3, so some caution is recommended in this regard."

- Documents from Yanagisawa's organization

Yanagisawa's Variant
During the climax of the series, Yanagisawa revealed that he had created a modified variant of the tentacle seed to grant him the powers of a tentacle creature while still keeping his human qualities, which worked by strategically implanting tentacles into his vital organs.

Yanagisawa possesses none of the side effects of the conventional tentacle seed, but in turn his body is vulnerable to any variant of an anti-sensei weapon.

Tentacle Mines
This form of tentacle weaponry worked by placing a live tentacle inside a container equipped with a sensor, which propelled itself at supersonic speed at a detected life form. These were not effective against tentacle creatures, but proved otherwise against humans; as shown with Aguri Yukimura.