Onyx was a longstanding, fully capable member of the League of Assassins, who eventually decided to retire from her life of murder to become a vigilante. She resides in Gotham City and is considered an ally of the Batman.
Onyx is highly trained in martial arts and served as a member of the League of Assassins before reforming and becoming a vigilante. When Onyx leaves the League she chooses a life in an Ashram Monastery, an order to which Green Arrow once belonged. The Ashram's Exalted Master is later killed by the renegade monk Lars, who intends to unravel the "Wisdom Key" that unlocks the powerful Book of Ages. Onyx sought Green Arrow's help to defeat the villain. In Ashram, Green Arrow and Onyx were defeated by Lars' minions, and the monk takes the Wisdom Key from Onyx. Upon opening the Book of Ages, however, Lars is vaporized.
Later, Onyx appears in Star City to battle a villain named Barricade. Barricade turns out to be Lars, who was revived by his underlings. The spell that had apparently killed him was reversed only as long as he remained in contact with the Book of Ages, but he can make the reversal permanent with possession of the Wisdom Key. In a battle with Onyx, Green Arrow, and Black Canary, Barricade again meets defeat and is reduced to a heap of bones.
After that, Onyx returns to monastery life but is called upon by Batman to serve as a bodyguard for Orpheus, and she works alongside Orpheus until his death. Onyx's notable appearances include the "Under the Hood" arc, in which she battles the new Red Hood, and the "War Games" crossover. Besides Catwoman, Onyx is the only independent vigilante Batman allows to operate freely in Gotham City.