these are forbidden jutsu because the usage of these jutsu will lead its user to death or needs human sacrifaction.
it's a life-trade jutsu, meaning you have to exchange your life in order to summon this Death God. anyone who uses this jutsu will end with absolute death. this jutsu has been used by Yondaime when he sealed Kyuubi into newborn baby Naruto. Sandaime also used this kinjutsu to seal Orochimaru's hand.
an excellent jutsu invented by the twisted-mind Orochimaru through human experiment. with this justu he could transfer his soul into a new fine body for 3 years to come, meanwhile the original owner's soul will vanish. so far Orochimaru had transferred his soul into 3 different bodies (not yet Sasuke).
revival-type jutsu which also used by Orochimaru and Nidaime. in order to revive a dead person, Orochimaru uses a medium (human) to transfer the dead soul. in the battle with Sandaime, Orochimaru revived the late Shodaime (1st) and Nidaime (2nd) Hokage by using his own underlings as the mediums. when the dead soul is released out the medium will die. the 3rd jutsu user is Kabuto, he's ables to revive Itachi, Sasori, Deidara, Kakuzu, and Nagato.
a genesis rebirth jutsu mastered exclusively by Tsunade, by storing some amount of chakra continuously into her forehead seal. this huge amount of chakra could be release and it will regenerate her body tissues in seconds. however, this jutsu will suck off her life span which means she turns older and die faster.
this forbidden jutsu is used by Sasori. his first experiment was none other than his own parents. basically, hitokugutsu turns human into puppets without losing their chakra and jutsu. for instance, Sasori killed the 3rd Kazekage and turned him into a puppet which still possesses his original jutsu and chakra.
what's the difference of Chiyo's tensei and Edo tensei? the answer is, Edo tensei needs a medium to reside the dead soul, yet Chiyo doesn't need any mediums. instead, she exchanged her own life to revive the dead person (Gaara) or blow a soul into puppet. in this case, it leads her to eternal sleep.
another immortal jutsu mastered by Hidan. this jutsu enabled Hidan to kill his opponents by suicide. however Hidan needs to taste the blood of his opponent beforehand. anything that Hidan do to his own body will happen to his opponent's body as well. for instance, if he stabs his own heart, his opponent will die but he himself will remain alive.