Satori

is a demonic, bird-like creature that appears in Naruto 5: Blood Prison.

Background
According to Gamabunta, the Box of Ultimate Bliss "brings forth", "actuates", and "ends" with Satori. Beyond that, like the Box, Satori's history is a mystery, save that it has been around since the time of of the Sage of the Six Paths.

During Gamabunta's talk with Naruto about Satori, a brief flashback is seen to what is most likely during the Era of the Rikudo Sennin, where Satori was unleashed on the world.

Appearance
The beast, Satori, is large, similar in size to Gamabunta and the Eight-Tails, with large wings. It is headless, with bones potruding from the mouth on its chest and is covered in black feathers. It also has a long thin tail, and its hands and feet are clawed and sickly thin to the point where it looks almost bone-like with skin attached.

Abilities
It can shoot its feathers and giant spheres at high speeds and appears to be very strong and a fast flyer. Due to the Box of Ultimate Bliss, its body can detect fear and blood lust, which causes it to instinctively dodge essentially all attacks from opponents who have any fear in them. This ability allowed it to dodge all of the attacks from the entirety of the Konoha 11 and Naruto Uzumaki, even while he was in Sage Mode.

Plot Overview
After Mui asked the box to release his son Muku, the boy walked out of the box and struck down his father. When Ryūzetsu, Muku's Childhood Friend, made herself known after Mui was struck down, Muku became conflicted and transformed into Satori. After transforming into the beast, he began attacking the prison, throwing inmates into the box, which led to his confrontation and battle with Naruto and his allies.

Trivia

 * It was thought that Satori had the ability to read the minds and hearts of his opponents in order to evade all of their attacks, but in reality its body detected certain emotions like fear, causing it to instinctively dodge their attacks.
 * Satori comes from a Japanese Buddhist term for enlightenment, meaning "understanding" and is based upon the yōkai of Japanese folklore.

Quotes

 * (To Naruto during their confrontation) "You may have mastered Senjutsu, but you've yet to free yourself from fear."
 * (Speaking of his experience inside the Box of Ultimate Bliss) "Can you imagine what it's like in this box? There's no exit and you can't destroy the box itself. In complete darkness, only one emotion exists within. It's pitch-black and heavy, seeping into every last cell of yours. That was the emotion that honed me. All emotions that gave me grief became ambiguous. And when I killed my father, it completely disappeared. Nothing's holding me back anymore, and that feels great..."