The woman who would be Myriad was murdered by Lex Luthor's clone, who was thought to be Lex Luthor's son, Lex Luthor II at that time. Myriad was originally Sasha Green, a martial arts instructor who was training Luthor. One day she beat Luthor in front of Supergirl and Lois Lane and they laughed at him. For that Luthor killed Sasha Green and dumped her body at a Lex San land fill.
He later gloats over Superman's coffin that he committed murder and the Man of Steel couldn't stop him or prove he did it! He's proved he's the superior one once and for all.
Sasha's body was later found and fed off of by the alien parasite Gemir, unintentionally bringing her back to life and granting her superpowers. These powers allowed her to literally become any person she touched. Unfortunately, each time she took on a new identity she would forget who she was before and ultimately made her a myriad of various people.
Luthor had Myriad captured and ordered an assassin of his, named Jillian Simmons, to take her and kill her while making it look like a helicopter crash. Myriad ended up touching Simmons during a struggle aboard the helicopter and had her kill herself, let the helicopter crash, and took her Simmon's identity.
She later teamed up with Hitman to destroy the Taker.
Her current whereabouts are unknown.
Powers and Abilities
Sasha Green was a proficient martial artist.
As Myriad, Sasha gained the ability to become whomever she touched. This generally causes her to gain not only the person's abilities, but also their personality, memories, and their appearance to a certain degree. The downside of these powers is that Myriad suffers amnesia after absorbing traits from another person and will typically behave like them.
Once she's touched a person, or made "contact" as she calls it, Myriad develops a mind link with the person. This link allows her to control them, but it also causes her to feel their pain. She will therefore make the person kill themself in order to sever this link.
Myriad acquired the abilities of a professional assassin like this.