Thousand Dragon was the gatekeeper of the Underworld Kingdom ruled by Gaia.
He had 1000 servants, who could transform into a large wolf's head as battle formation.
Ginga Nagareboshi Gin
He tries to trick Gin to jump inside and get killed, but Noroi comes in the way and sacrifices himself by jumping into the wolf's mouth. Noroi did not die yet because the wolf spat him out, but Noroi died later from the injuries he sustained. Retsuga created a hurricane and blew all 1000 of Thousand Dragon's servants away, but did not get Thousand Dragon himself, so it pulled Reima into the underground world of Gaia. According to Reima, he gave Thousand Dragon hit before the wolf disappeared and was not seen again.
See more pictures in Thousand Dragon/Gallery.