Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style, consisting of two Four Symbols Seals, was used by Minato Namikaze to help Naruto Uzumaki access the chakra of the Nine-Tailed Demon Fox sealed within him. To perform the technique, the user must first summon a ceremonial throne and place the target on it. The seal was not automatically visible on Naruto's belly, but became perceptible whenever he drew chakra from the Nine-Tails.

Whenever Naruto and the Nine-Tails conversed, it occurred within Naruto's subconscious, which was depicted as a basement of a boiler room or a sub-level of a power plant, with pipes running across the ceiling and the floor being flooded over with water. On the entrance gate of the Nine-Tails' cage, there was a parchment of paper that said, which covered the lock that kept the Nine-Tails in.

Minato also sealed his and Kushina's remaining chakra into Naruto with this seal. Minato appeared in Naruto's subconscious when Naruto transformed into his eight-tailed form, and restored the seal to its original strength, but he could only do it once. Kushina also appeared and assisted Naruto when he later attempted to control the Nine-Tails.

Key
Because the seal itself would naturally weaken over time, Minato created the key, on the abdomen of the scroll toad Gerotora, that can either strengthen or completely remove the seal.

During Naruto's two and a half year training with Jiraiya, Jiraiya would use the key to suppress the Nine-Tails when necessary, but for once he used it to weaken the seal for just a short time, so he could have Naruto try to filter out the Nine-Tails' influence when he tapped into its chakra. Unfortunately, it instead unleashed the four-tailed form.

During Naruto's training to control the Nine-Tails at the Falls of Truth, the seal was destroyed when the Nine-Tails used its Tailed Beast Ball to try and blast Naruto after he used the key to open the seal's gate. However, after the Nine-Tails' chakra merged with Naruto, he was able to imprison the Nine-Tails itself with a new, stronger seal that consists of massive interlocking torii.

Trivia

 * As part of the Uzumaki clan's fūinjutsu, the Eight Trigrams Sealing Style, the Four Symbols Seal, and the Dead Demon Consuming Seal all bear the same spiral mark.