WARNING: This article contains major spoilers for "A Secret Kept From All The Rest," the penultimate episode of Batwoman Season 1.
Batwoman just paved the way for Batman antihero Azrael to appear in the Arrowverse.
In "A Secret Kept From All the Rest," the penultimate episode of Season 1, Alice began kidnapping intelligence experts around Gotham and forced them to try and decode Lucius Fox's journal. No one could crack the code except Lucius' son Luke, who refused to do so in order to save Kate from certain death. Nevertheless, Kate arrived later with the key to Lucius' journal: a pair of glasses that automatically decoded the pages for the wearer.
To prove she wasn't bluffing about the glasses, Kate handed them to Alice. When she used them, the camera cut to show an excerpt from the journal. The relevant passage appears in full below:
During the resurrection of Ra's al Ghul, Batman... suit of armor from Talia al Ghul, The Suit of Sorrows. According... its legends, it can impart strength and speed of its wearer but also would completely corrupt anyone whose heart and soul is not pure. At first, the Dark Knight was dubious of the legend, but eventually experienced an aggressive behavior while wearing the armor during patrols. Batman later learns from a member of The Order Of The Pure, a splinter faction of The Order Of St. Dumas, that the armor once belonged to a knight named Sir Geoffrey de Cantonna, who massacred hundreds of people in an alpine valley in 1190. The Suit of Sorrows becomes one of the trophy displays within the Batcave, to remind the Dark Knight that he must be ever vigilant not only in his crusade against crime, but also himself. The new Azrael takes up wearing the suit eventually.
Lucius' journal doesn't beat around the bush. This passage lays out the relevant aspects of Azrael's mythology and even references him by name, which likely isn't a coincidence. In the same way The Flash's newspaper from the future predicted the "Crisis on Infinite Earths" crossover, the journal appears to be setting up Azrael's Arrowverse debut.
Azrael burst onto the DC Comics scene in 1992's Batman: Sword of Azrael #1 by Denny O'Neil, Joe Quesada and Peter Milligan. Though multiple people have taken up the Azrael mantle, Jean-Paul Valley is best known as the antihero, who has violent tendencies despite his good intentions. He belongs to the Sacred Order of St. Dumas, a group of assassins. They sent him on mission in Gotham, where he eventually defied the Order and became Batman's ally. He even took up the Dark Knight's title for a while.
That last aspect of Valley's character may be a point of contention with Kate. After all, Kate's Batwoman journey began when she took up Batman's mantle in Bruce Wayne's absence. Since Batman and the Order of St. Dumas are familiar with one another, Valley may come to town looking for the Suit of Sorrows and perhaps even clash with Kate in the process.
Notably, the passage also refers to Ra's and Talia al Ghul. Both characters have previously appeared on Arrow as leaders of the League of Assassins. In the Arrowverse's pre-"Crisis" mythology, Oliver Queen disbanded the League after he defeated the previous R'as al Ghul to become the Demon's Head himself. Talia appeared later with an ax to grind over her father's death, but eventually became an uneasy ally to Team Arrow. She even attended Oliver's funeral in the Arrow series finale.
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.