WARNING: This article contains major spoilers for "A Secret Kept From All The Rest," the penultimate episode of Batwoman Season 1.

In Sunday night's episode of Batwoman, "A Secret Kept From All The Rest," Alice deciphered Lucius Fox's journal, which explained how to kill Batman. In the final moments of the episode, Alice told Mouse to bring her Tommy Elliot/Hush so that he could tell her "where to find a little green rock called Kryptonite," as it's important to killing Batwoman.

While it's not yet clear how Superman and Supergirl's infamous weakness factors into what Lucius wrote, Batwoman creator and showrunner Caroline Dries explained in an interview with EW why Kryptonite is appearing in the more grounded superhero series and how "Crisis on Infinite Earths" helped her formulate the mineral's introduction.

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"I’ve talked about this before, but the only way I was really comfortable in doing the ["Crisis on Infinite Earths"] crossover was if we could find ways to keep our world still feeling really grounded," Dries said. "One of the most wonderful aspects of the crossover was the Kate Kane-Kara Danvers friendship. Part of that storyline was centered around this piece of Kryptonite, and it just kind of represented this bond between them; it was just so much bigger than an alien mineral [because] it represented trust and a rhyme to Bruce Wayne and Clark Kent’s relationship from the comics. So, I wanted to keep that in our world. Even though it’s an alien rock, we have introduced it on our show."

Dries further explained how the show features high-tech gadgets, but thought about the possibility of something else that could pierce the Batsuit. "So I was like, 'Yes, Kryptonite!'," Dries said. "There are very few [rocks] in our ether."

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Dries concluded by admitting that this move was a way for her to bring the fantastical elements from the larger Arrowverse into Batwoman while using the show’s own language. "I sort of didn’t want to do the crossover, which was so successful, and just pretend like that didn’t happen," Dries explained. "What happened was that this relationship came out of it and that’s what we can glob onto."

Created by Caroline Dries and developed by Berlanti Productions and Warner Bros. Television, Batwoman stars Ruby Rose, Rachel Skarsten, Meagan Tandy, Camrus Johnson, Dougray Scott, Elizabeth Anweis and Nicole Kang. The series airs Sundays at 8 p.m. ET/PT on The CW.