Complex sentence: It is finally discovered in the Season Finale "Arctic", that the real Kara never made it back to Earth and is trapped in the Phantom Zone, and that Brainiac had been impersonating her.
Split into simpler sentences: It is finally discovered in the Season Finale "Arctic", that the real Kara never made it back to Earth. Brainiac trapped her in the Phantom Zone, and has had been impersonating her.