As recently revealed, Reign (Odette Annable) is not the only Worldkiller, as there are at least two more, including Purity (Krys Marshall), with whom Kara (Melissa Benoist) will soon tussle. However, Supergirl’s battle with the second Worldkiller will be quite different from the knock-down, drag-out brawl with Reign that resulted in Kara’s coma. In fact, Kara will attempt to use different methods of interrogation in hopes of getting through to Purity.

“Supergirl is caught off guard when she realizes that Purity is less alien than she actually thought,” Annable tells EW. “So there’s an interesting relationship that they have there, and when Reign meets Purity as well I think she’s also a bit caught off guard. But for Reign, Purity is an ally and she’s a sister and she isn’t at all threatened by her. Purity is someone that Reign knows can help her and that she’s someone that she can guide and train. So, for Reign, it’s completing her circle and making her more powerful.”

What will ultimately make Reign even more powerful is the impending introduction of the third Worldkiller, so far only known as Pestilence. While Annable is coy to say how Pestilence may be different from the other Worldkillers, the actress notes, “I’m assuming that she or he will complete the other two and once that happens, their power together will be unstoppable.”

In the comics, five Worldkillers survived the destruction of Krypton, including Reign, who was their leader. But there now being a triumvirate of Worldkillers on the CW series was a pleasant surprise to Annable. “I really didn’t know [there would be more],” she says. “I knew that there were Worldkillers, obviously, in the comic books, but I didn’t know if they were going to be shown in the series or in the arcs that we’re showing in this season. So, I was very happy to know that I had a sister coming along and another one coming along very shortly as well.”

But while this episode — the last until April 16 as DC’s Legends of Tomorrow will now air in Supergirl’s time slot — will be epic in scope, Annable teases it’s also quite emotional as Sam is still unaware of her lethal alter ego. “Sam, at this point in her journey, I think she’s really, really desperate,” Annable says. “She is desperate to find out what’s going on with her. She’s finally coming to terms that there is something going on, and her main goal is to protect her daughter and make sure that relationship is okay and make sure Ruby is okay. So, needless to say, I think Sam is pretty fearful right now.”

“The beauty of Supergirl is that every single episode feels sort of like a movie,” Annable continues. “They’re all very ambitious episodes, and they’re all so beautifully shot, and it just feels big. The fight scenes are definitely epic in this episode, but also Sam’s arc is particularly heartbreaking, because she really is starting to realize that there is something devastating going on, and she can’t control it, and she feels like she’s crumbling, and if she doesn’t figure it out then I don’t think she even wants to admit what might happen.”

Supergirl airs Mondays at 8 p.m. ET on The CW.