He obeys the Demon King, Lord Mei-Oh.
In Tenchu: Stealth Assassins he makes his first on screen apperance in the mission: Rescue the captive ninja.
He is killed by Rikimaru in the mission: Free the princess, before you kill Mei-Oh.
In Tenchu: Birth of the Stealth Assasin,he was disguised as Suzaku, the red sparrow one of the lord of the burning dawn. Rikimaru defeated him in a second large ship before Rikimaru against Lady Kagami.
In Tenchu Wrath of Heaven Rikimaru spots Onikage after killing the merchant Nasu and later learns that a powerful sorcerer called Tenrai brought him back to life to serve him and Lord Mei-Oh, he is defeated by Rikimaru but is revealed to be an imposter with the real Onikage appearing and cutting Rikimarus throat. Using Lord Mei-Oh's sword he tries to open a portal in the limestone cavern to revive his master however Rikimaru jumps out the portal revealing the Rikimaru that he killed was his shadow. The pair begin to fight with Rikimaru emerging as the victor, several ninja attack Rikimaru allowing Onikage who was playing possum to cut Rikimarus right eye before vowing to kill him and flees the limestone cavern. During Ayame's story after she escapes Amigi castle Onikage appears and attacks her with Rikimaru showing up to fight Onikage, the player can choose to fight him themselves or allow RIkimaru to do so. When Rikimaru attacks Tenrais fortress he encounters Onikage waiting for him to arrive, if the player fixed Rikimarus sword with steel from Lord Mei-oh's sword Onikage reveals that this was a trap that he had set up with the consequences draining the swords power and giving Onikage the edge. Even with the swords power diminished or if the player fixed the sword with regular steel Rikimaru defeats Onikage who apologizes to Lord Mei-Oh and dives into a large pit with an explosion erupting from the location signaling his death. However after Rikimaru kills Tenrai the figure of Onikage rises up and informs the spirit of Lord Mei-Oh that his time to return is at hand.
Onikage returns during the game Shadow Assasins, where he pretends to be Lord Goda's adviser but once Rikimaru tries to kill him he shows his true nature and fights Rikimaru. After another defeat he uses princess Kiku as a shield and goads him into attacking however princess Kiku urges him to do so. RIkimaru fights his urge but decides to run through Kiku killing her and Onikage, although he dies in the extended ending it shows Ayame speaking in Onikage's voice as if he had somehow possessed her.