Six Paths Ten-Tails Coffin Seal

The is a sealing technique used by Obito and Madara Uchiha to seal the Ten-Tails into themselves, thus becoming the beast's jinchūriki. After a Rinnegan user performs the required hand seals while in close proximity to the Ten-Tails, the beast is quickly absorbed into the user's body.

Overview
It is noted to be difficult to control the tailed beasts in this form, and if the user does not have strong willpower — either inherent or acquired — the user can have a psychological collapse. If they are able to find that strength of will, they'll obtain the power of a "god that shapes the land", essentially acquiring the power of the Sage of Six Paths.

Upon using the seal to become the Ten-Tails jinchūriki, Obito achieved two forms; in his initial form, his skin tone turned grey and his right half became scaly, he gained ten protrusions as well as the markings of the Sage of Six Paths on his back, and he obtained two Truth-Seeking Balls which floated behind his hands. After gaining control over the Ten-Tails, the protrusions on his back changed to resemble clothing, he gained a pair of horns on his head — the right being larger than the left — and his Truth-Seeking Balls increased to ten. In the anime, when Obito sealed the Ten-Tails, a circular shell encased his body and shattered once the transformation was complete.

When Madara used this seal, his skin became grey and his hair turned white (taupe in Naruto: Colour Edition). Instead of possessing clothing-like flesh, Madara was instead cloaked with a complete outfit composed of chakra, with a pattern of six black magatama below his neck and the markings of the Six Paths Senjutsu on his back. A forehead protector-like appendage also emerged from the left side of his forehead, with an upward curve on its right side and a single horn on its left side. Like Obito, Madara obtained ten Truth-Seeking Balls. After using the seal to absorb the tree left behind by Obito, Madara gained additional magatama markings along the bottom and sleeves of his cloak.

Abilities
As the jinchūriki of the Ten-Tails, the user gains immense increases in their physical capabilities and are able to completely overwhelm their powerful adversaries. Even when Obito initially had no control over himself, he was able to destroy the Hokage's barrier as well as Hashirama Senju's gates with ease, as well as overwhelm all four reincarnated Hokage. This power amplified greatly upon having complete control of the Ten-Tails.

With it, Madara was able to fight on par against Might Guy using all Eight Gates and survive the latter's final attack. His speed was also greatly increased, able to counter a combination of individually fast techniques. Madara's regenerative powers and durability are also greatly enhanced, able to survive the left side of his torso being obliterated or being bisected and soon after completely restore himself. With his immense regenerative powers, Madara claimed he had achieved complete immortality.

Trivia

 * When Obito revealed his plan at the Kage Summit, he stated that there was a fūinjutsu used to create jinchūriki that was passed down through the stone tablet beneath the Naka Shrine. While it is unclear whether this was the jutsu used by Hagoromo to seal the Ten-Tails into himself, it is possible that this was the technique mentioned by Obito.
 * The Truth-Seeking Balls created through this technique are said to be similar to Tailed Beast Balls.