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 reflect on the invoker's own body.

After the souls are released, the person is free to do as they please with the souls. Orochimaru used this technique to free the souls of the Hokage along with that of his arms and then reincarnate them.

Trivia

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