Kara Zor-El lives in an isolated Kryptonian community named Argo City, in a pocket of trans-dimensional space. A man named Zaltar allows Kara to see a unique and immensely powerful item known as the Omegahedron, which he has borrowed without the knowledge of the city government, and which powers the city. However, after a mishap, the Omegahedron is blown out into space. Much to the distress of her parents, Kara follows it to Earth (undergoing a transformation into "Supergirl" in the process) in an effort to recover it and save the city.
On Earth, the Omegahedron is recovered by Selena, a power-hungry would-be witch assisted by the feckless Bianca, seeking to free herself from her relationship with warlock Nigel. Whilst not knowing exactly what it is, Selena quickly realizes that the Omegahedron is powerful and can enable her to perform real magical spells. Supergirl arrives on Earth and discovers her powers. Following the path of the Omegahedron, she takes the name Linda Lee, identifies herself as the cousin of Clark Kent, and enrolls at an all-girls school where she befriends Lucy Lane, the younger sister of Lois Lane who happens to be studying there. Supergirl also meets and becomes enamoured with Ethan, who works as a groundskeeper at the school.
Ethan also catches the eye of Selena, who drugs him with a love potion (which will make him fall in love with the first person he sees for a day); however, Ethan regains consciousness in Selena's absence and wanders out into the streets. An angry Selena uses her new-found powers to animate a construction vehicle which she sends to bring Ethan back, causing chaos in the streets as it does so. Supergirl rescues Ethan and he falls in love with her instead while in the guise of Linda Lee.
Supergirl and Selena repeatedly battle in various ways, until Selena uses her powers to put Supergirl in an "eternal void" known as the Phantom Zone. Here, stripped of her powers, she wanders the bleak landscape and nearly drowns in an oily bog. Yet she finds help in Zaltar, who has exiled himself to the Phantom Zone as a punishment for losing the Omegahedron. Zaltar sacrifices his life to allow Supergirl to escape. Back on Earth, Selena misuses the Omegahedron to make herself a "princess of Earth", with Ethan as her lover and consort. 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. Free from Selena's spell, Ethan admits his love for Linda and that he knows that she and Supergirl are one and the same, but knows it is possible he may never see her again and understands she must save Argo City. The final scene shows Kara returning the Omegahedron to a darkened Argo City, which promptly lights up again.
Output: What is the "eternal void" to which Selena attempts to banish Supergirl called?