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."

Upon using the seal to become the Ten-Tails jinchūriki, Obito achieved two forms; the first which he achieved in his initial form, his body became scaly, he got the Rinnegan symbol and the nine tomoe on his back, and he possessed only two Truth-Seeking Balls which were attached to his hands. After gaining control over the Ten-Tails, he possessed a chakra cloak behind him, gained small horns on his head with one over his Sharingan being bigger than the one over his Rinnegan and his Truth-Seeking Balls increased to ten. In the anime, when Obito sealed the Ten-Tails, a circular shell encased his body which shattered once the transformation was complete.

For Madara, after using this seal, his skin became grey and his hair turned taupe-colour (white in the anime). Instead of possessing clothing-like flesh, Madara was instead cloaked with a complete outfit composed of chakra, with a pattern of six magatama below his neck and a pattern similar to Hagoromo's on his back. He also gained a forehead protector-like horn which emerged from the left side of his forehead, and had an upward curve on its right side and a single horn on its left side. He was able to create ten Truth-Seeking Balls.

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.

Despite both Madara and Obito gaining enormous increases in power, Minato Namikaze noted that Madara's power in this form was greater than Obito's. With it, Madara was able to fight on par against 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 Tobi revealed his plans at the Kage Summit, he stated that there was a fūinjutsu, used to create jinchūriki, passed down through the Uchiha Stone Monument. While it's unclear whether this was the jutsu used by Hagoromo to seal the Ten-Tails into himself, it's highly likely that this was the technique mentioned by Tobi.
 * In the manga, unlike other fūinjutsu techniques used to keep a tailed beast sealed within a host, this technique seems to allow the user to manifest a portion of their tailed beast outside the jinchūriki's body as a separate entity as demonstrated by Obito.