Demon Chaos, known in Japan as Ikusagami (戦神, lit. God of War), is in vein of the well known Dynasty Warriors series. It is set in feudal Japan in the 16th century and revolves around a priestess who has been given eternal youth until she exterminates all the demons. She has been given charge of a mystical beast from the gods, Inugami, which is controlled by the player. The unique aspect of the game is that up to 65,535 enemies can be on-screen at once.
Source: Wikipedia, "Ikusagami", available under the CC-BY-SA License.
Gameplay is basically the hack and slash controls from Dynasty Warriors. While you fight as an individual using basic thrust and parry controls, you also have an allied army to assist in fighting hordes of enemies. You can set up health areas to support your army and fighting the monsters as they attack.