Dead Demon Consuming Seal: Release

This technique involves invoking the Shinigami using a mask from the Uzumaki Clan's Mask Storage Temple. From there the summoner is possessed by the Shinigami allowing them control over it, they can then force it to cut its own stomach resulting in the release of the souls sealed inside. Due to the possession however, the same damage will appear on the invoker's own body.

Orochimaru used this technique to restore the severed part of his soul and free the souls of the previous Hokage, allowing him to reincarnate them with the Summoning: Impure World Reincarnation technique.

Trivia

 * Though this technique is not named by Orochimaru personally, the kanji for its name are readable in the scroll which details the technique.