Asura was one of the twelve demons that made contact with Kagemitsu Daigo. That had appeared as a statue ever since in Episode 12. It wasn't long until make it actual physical appearance in Episode 22 possessing Mutsu, Hyogo and Tahomaru after pleasing and made a pace to him to restore their body part to after being sliced by Hyakkimaru.
Appearance
Asura took the form of the three head and six arms with three eyes each on its face.