He is the demon king and Onikage is Mei-Oh's right hand man.
He dies by Rikimaru (or Ayame) in the mission: Free the princess.
In Tenchu: Wrath of Heaven he is mentioned by Rikimaru in mission: Find the Shichishito sword. In Ayames story Tenrai offers himself to Mei-Oh after being defeated and absorbs his power and two of the jewels he had collected but Ayame still defeats and kills Tenrai. In Rikimarus story however Tenrai demonstrates the power that Mei-Oh can offer and uses it to attack RIkimaru, after an intervention from fellow azuma ninja Ayame he musters the strength to kill Tenrai. After Rikimaru defeats and kills Tenrai the previously thought deceased Onikage appears and converses with the impatient spirit of Lord Mei-Oh stating that his time to return has finally come.