Shinigami

The is the spectral entity summoned through the usage of certain Uzumaki clan fūinjutsu.

Background
The Shinigami was first summoned by Minato Namikaze, the Fourth Hokage, as a last resort, with the intention of sealing the Nine-Tailed Demon Fox into his infant son. However, as he wasn't able to seal the entirety of Kurama's chakra into the infant, due to the beast's size. Using the Shinigami, Minato was able to seal the yin half of the tailed beast within himself, before going on to use another sealing method to seal the yang half 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, as well as purple-hued skin. It is draped in a large white kimono 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 spread across its arms.

Abilities
The Shinigami's primary 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 use of a special mask stored within the Uzumaki clan temple outside of Konohagakure, the contents of its stomach can be released.

Konoha Crush
During the Konoha Crush, 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.

Fourth Shinobi World War: Climax
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 himself, to release the trapped contents inside the Shinigami.