Selena, impatient to be the most powerful sorceress in the world discovered the Omegahedron from Argo City and planned to use it for evil by taking over the world just like Lex Luthor did.
She has also captured Supergirl's love interest, Ethan in her evil palace and sends Supergirl to the Phantom Zone where she loses her Kryptonian powers and became powerless. But she escaped from the prison dimension with help from Zaltar. Emerging from the Phantom Zone through a mirror, Supergirl regains her powers and confronts Selena, who uses the Omegahedron's power to summon a gigantic shadow demon.
The demon overwhelms Supergirl and is on the verge of defeating her when she hears Zaltar's voice urging her to fight on. Supergirl breaks free and is told by Nigel the only way to defeat Selena is to turn the shadow demon against her. Supergirl quickly complies and begins flying in circles around her, trapping her in a whirlwind.
Selena is attacked and incapacitated by the monster as the whirlwind pulls Bianca in as well. The three of them are sucked back into the mirror portal, which promptly reforms, trapping them all within forever.
Selena was a cruel and ruthless woman who would stop at nothing to achieve her dream of world conquest even if others had to suffer. She was also extremely vain, thinking of herself as the ultimate sorceress. As Supergirl put it she had no friends and treated everyone as if they were made to serve her. She even tried to kill her only friend Bianca when she tried to escape.
Power of Shadow...
Now I am really upset!
~ Selena after her first fight with Supergirl.
(Supergirl: The Omegahedron Selena, I want it.) Well then Supergirl, you shall have it!
~ Selena and Supergirl
(Supergirl: You have no friends Selena, you treat everyone as if they were put on this Earth to serve you.) More or less I think they were. You included!
~ Selena talking with Supergirl
A promotional picture of Selena with her arch-enemy Supergirl