Comic Book History Of Reign
The season 2 finale for Supergirl showed us that a third ship was launched from Krypton before the planet exploded. Speculation ran wild as to who it could be, ranging from Brainiac to Doomsday and a dozen other ideas. But the producers quickly announced that Odette Annable (Banshee) would be joining the cast as the season 3 villain Reign. Current comic readers are likely to know who Reign is, but for the TV only fans or readers who may not be up on the more recent DC Universe, here is the comic book history, albeit brief, of Reign.
She first appears in Supergirl Vol 6 #5 in 2012 from creator Michael Green, Mike Johnson and Mahmud Asrar. She is described as being a Worldkiller and coming from Argo City. She was a biological weapon created on Krypton along with four other Worldkillers by Zor-El, but then outlawed by the Science Council. Zor-El would later regret making the monsters. Reign on the others escaped the destruction of Krypton, but had no knowledge of how she came to be, just that she is tied to both Krypton and Earth.
She followed Supergirl to the remains of Argo City, beating Kara Zor-El severly but getting none of the answers she was looking for. She left Kara for dead and headed back to Earth to conquer the planet. But Supergirl followed and confronted Reign and three of the Worldkillers. Though exhausted, Kara was able to injury one of the Worldkillers, causing Reign to stop their rampage and retreat, fearing losing one of her partners.
There's not a lot to speculate on in that brief history. The TV series could follow the basic story idea, have Kara travel back to the remains of Krypton and be confronted by Reign, then leading the fight back to Earth. They could have her attack alone or with the other Worldkillers which could be a way to better utilize J'onn J'onzz, Superman and possibly Mon-El. Or they could go for more of a Doomsday style story with Reign going on an rampage and Supergirl dying to stop her. The character's history is vague enough that the writers can really do what they want.
We do know from THR.com that Robert Gant will reprise his role as Zor-El, who is the father of both Supergirl and Reign in a way. And that the costume will be different than in the comics, which makes perfect sense as you can see above, the Annable would have to be in make-up constantly to be accurate to the comic.