Shinigami

The is the spectral entity summoned through the usage of a unique fūinjutsu.

Background
Minato Namikaze, the Fourth Hokage, first summoned the Shinigami, as a last resort, with the intention of sealing Kurama into his infant son. However, as he wasn't capable of sealing Kurama's entire chakra, due to its size, he was forced to seal only the yang half. Using the Shinigami, Minato was able to seal the yin half of the tailed beast within himself, going on to use another sealing method to seal the other portion of the fox within his son.

Appearance
The Shinigami is a translucent gaunt spectre with a demonic visage. It is much larger than an average human, possessing long, shaggy white hair, from which two red horns protrude, and dark greyish blue-coloured skin. It is draped in a large white garments and carries a set of prayer beads. When not using it, the Shinigami is almost always seen with a tantō in its mouth. When removed, however, its mouth is revealed to be full of sharp, jagged teeth, and a very long, almost serpentine tongue.

When sealing a target, a series of cursed seals will spread across its arms.

Abilities
The Shinigami's main strength lies in its ability to forcibly extract the souls of both its summoner and its summoner's target, cut them free with its tantō, and seal them within its belly. It does this through the Dead Demon Consuming Seal technique, primarily used by Minato Namikaze and Hiruzen Sarutobi. The amount of the victim's soul that can be extracted and sealed is dependent on the summoner's chakra level proportionally to that of the victim's; regardless of the amount of success, the Shinigami will seal away the summoner's soul upon the technique's evocation.

Through the use of a special mask, stored within the Uzumaki clan temple outside of Konohagakure, the contents of its stomach can be released.

Invasion of Konoha Arc
During the invasion of Konoha, the Shinigami appeared as Hiruzen executed the Dead Demon Consuming Seal technique in an attempt to seal away the First and Second Hokage's souls. As he bid a heartfelt goodbye to his teachers, he turned his attention to Orochimaru and revealed the technique's double edge as he must sacrifice both his soul and those of his predecessors to the Shinigami. Intending to have Orochimaru join them in the spectre's stomach while throwing aside his former pupil's Kusanagi, Hiruzen attempted to seal Orochimaru's soul as well. After an hour of struggling, Hiruzen came to accept that his old age had left him too weak to completely seal Orochimaru away. Seeing that completely removing Orochimaru's soul was impossible, yet his desire to redeem himself for allowing his former student's escape, Hiruzen decided to instead have the Shinigami take Orochimaru's arms to rob him of using ninjutsu as his way of fittingly punishing Orochimaru for his obsession with power.

Ten-Tails Revival Arc
In an attempt to free the former Hokage's souls from the Shinigami's stomach, Orochimaru led Sasuke, Suigetsu, and Jūgo to the Uzumaki Clan's Mask Storage Temple to pick up a mask from the ruins, and then took the group to the underground hall in the Naka Shrine. Orochimaru proceeded to don the mask which invoked the Shinigami to possess his body. Orochimaru then cut open the Shinigami's stomach, a wound reflected on his own body, to release the trapped contents inside the Shinigami.