Susano'o

The Susano'o (須佐之男, "Those Who Serve to Help") were spiritual beings that were studied in the that lived in the land prior to the founding of Soul Society. Named after the kami itself, they were said to have served as agents of Izanagi to maintain the balance of life and death throughout the worlds, having done so until their eventual decline at the hands of the Shinigami.

History
The Susano'o were wiped out by Genryūsai Shigekuni Yamamoto and his Divisions during the ending stages of a civil war between the two groups, with several of the former joining the ranks of Yamamoto as Shinigami as the others perished in the battles.

Shōkangakki
The Shōkangakki (召喚楽器, "Summoning Instrument") were medallions used by the Susano'o to call forth spiritual beings from the Well of Souls, allowing them to be used at the summoner's whim.