There are a startling number of disastrous techniques used in Naruto, and most require too much power and precision for the average ninja to use. Some of them are so devastating - or so immoral - that the greater ninja community officially banned them. These are called kinjutsu, and one particularly damaging example is Night Guy, which unleashes a huge blast of power that can destroy any opponent, but it also destroys the user's body. Living Reincarnation is another; it allows Orochimaru to transplant his soul into someone else's body, effectively vanquishing them.
Forbidden jutsu in Naruto can get pretty disturbing, so those with a weak stomach may want to tread lightly.
To use Summoning: Impure World Reincarnation, the user must first get their hands on a piece of DNA from the person they wish to reincarnate. Often, this is accomplished through grave-robbing, so that would be the first reason for banning it. The second reason is that the soul needs a new vessel - which means that a human being must be sacrificed for it to work. The user must smear the DNA on a scroll, which causes the living sacrifice's body to be taken over.
Originally, the technique created reincarnations that were far less powerful than the original and lacked any personality whatsoever. Kabuto was able to make some tweaks to the technique and fixed those problems, resulting in massively powerful clones who acted like their former selves, but were still forced to obey the commands of the technique's user.
Naruto fans may remember one of the anime's more disturbing scenes, which involves Orochimaru slithering around on the ground in what looks like a pile of chewed-up gum.
With this technique, the user can make their mind immortal, and transfer it to another body. He shows his true form, a giant white snake composed of multiple smaller snakes, then swallows his target to transport them to another dimension, where he funnels his soul into their body.
After this, he alters the new host's body to look like him. Orochimaru has done this multiple times in the pursuit of greater powers and indefinitely preserving his life. He has also tried to do it to Sasuke, but perished (temporarily) in the process.
Casualty Puppet may be one of the most disgusting kinjutsu in the world of Naruto. This technique allows the user to absorb a recently deceased body, gaining control over not only its movement, but also over any techniques the former living person could use.
This technique often causes mental instability, which makes it difficult for the user to use the bodies for whatever their original purpose may have been.
The Jar of Poison Technique is used primarily by Torune Aburame, a relative of Shino's. Though most techniques used by the Aburame clan are gruesome, this one is truly exceptional. Torune sends out venomous insects called rinkaichu, then gathers the corpses of infected people in a small, enclosed barrier. The bodies eventually give off a cloud of poisonous gas that sets off an explosion, sending swarms of insects and gas across the area.
The technique was banned because it required multiple bodies to use, and the explosion and poison gas were impossible to control.