Earth Grudge Fear (
Official Version)
This was a forbidden jutsu that once belonged to the Hidden Waterfall back during the early days of its creation. It was invented by a medical ninja who specialized in using medical-chakra infused threads to stitch up wounds. It was effective but primitive and not a well-known technique in the wider world. Eventually they developed a jutsu granted their body the ability to produce organic tentacles almost without limit and manipulate them with chakra to change their length and form at will to adapt to any circumstance. This allowed them to pull themselves together from any not immediately fatal injury without even needing hand-seals, and to stitch together their patients without needing medical equipment.
Kakuzu was the shinobi who was able to perform this kinjutsu on himself to grant him that very ability. Through his life of warfare, he discovered that he could steal the heart of another human being and assimilate it with his tentacles which allowed him to connect his chakra system to the chakra generated by the heart. The heart retains the strongest chakra nature of the original owner which Kakuzu could use as a shortcut for generating elemental chakra by channelling his chakra through theirs. This discovery led into another when he decided to steal the hearts of more of his enemies and learned that he could stock up to five hearts, ideally one for each element. Thus Kakuzu could cast any jutsu of the five nature transformations that he possesses knowledge of.
In addition, each heart holds a great proportion of the original owner's chakra, increasing his chakra reserves to the point that Kakuzu has almost as much chakra as five people combined. The more chakra the original owner held then the more chakra Kakuzu had access to. It even became clear that his stolen hearts were able to sustain his life force and retard his gaining process so he know that as long as he had a beating heart, he was able to stay alive.
This made him a very dangerous individual and the most prized fighter in his village. That's the reason that he was chosen to eliminate the First Hokage. It was believed that not only would he have the greatest chance of the defeating the man, but that by stealing his heart he would give his village a potent weapon against the Tailed Beasts.
Kakuzu only survived that battle by faking his death, taking advantage of his hearts' ability to move independently from him and return to his temporarily-lifeless body. After he was punished by his village for his failures, he stole scroll containing the forbidden jutsu and restored his strength by carving out the hearts of the village elders before leaving.
In his following years as a bounty hunter, Kakuzu's skill with Earth Grudge Fear only grew. He took the jutsu to a new level, developing a method to permanently convert his own body cells into those tentacles. His tentacles can perfectly simulate most of the functioning of any other cell, even including the neurons of the brain, turning him into basically a mass of prehensile tentacles wearing human skin. The only weakness of the tentacles is that they can't generate chakra, so he must retain a normal human heart.
More than that, he learned of a way to give his hearts even greater independence. Kakuzu would convert his stolen heart into masks sewn to his body. These hearts have sentience which means they can be released in the form of a monstrous body of tentacles. They can move and attack independently, and each can release jutsu belonging to its respective element from the mask, even using them simultaneously in devastating combinations. But since they are not capable of hand-seals or being on the same level as clone, each mask can only release their elemental chakra as a single, specific jutsu by themselves. However Kakuzu is still able to control the masks so by performing the hand-seals for them, they are able to produce a wider variety of jutsu.