SPOILER WARNING: This article contains major spoilers for "Trinity," the latest episode of Supergirl.
Supergirl's friend Lena Luthor just created a dangerous new weapon. In "Trinity," the latest episode, she revealed to James Olsen that she'd discovered a way to make Kryptonite.
"Trinity" found Lena dealing with the fallout from last week's episode, where Supergirl and the DEO discovered she had been researching the connection between Sam and Reign for the past three weeks. Lena admitted that she was able to hold Sam's alter ego Reign captive by using Kryptonite. This fact drove a wedge between Sam and Supergirl, who had been good friends since Lena arrived in National City in Season 2.
Initially, Lena told Supergirl that she had inherited the Kryptonite from her brother Lex, but the truth came out at the end of the episode. James came clean to Lena about his double life as the Guardian. In return, Lena told James the truth. She hadn't discovered the Kryptonite in Lex's stash; she had actually created it.
Up to this point, Supergirl and the DEO thought that all the Kryptonite on Earth had been destroyed. The fact that Kryptonite can be created synthetically means that Kryptonite will continue to be a threat on the show. Worse, if someone else realizes that Kryptonite can be created in a lab, the Girl of Steel may find herself in a lot more trouble than she ever expected. However, this could be the key to stopping Reign, as large quantities of Kryptonite seem to affect her too.
Airing Mondays at 8 p.m. on The CW, Supergirl stars Melissa Benoist as the Girl of Steel, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen, Chyler Leigh as Alex Danvers and Jeremy Jordan as Winn Schott, with appearances by Calista Flockhart as Cat Grant and Tyler Hoechlin as Superman.