A lot has happened in the month since it was announced that Katie Cassidy would guest star on an episode of "The Flash." Most importantly, and most spoilery if you're not up to date on "Arrow," Star City's resident Black Canary died and was laid to rest in the most recent episodes of "Flash's" companion series.

But that's not the end of Cassidy's involvement in the "Arrow"-verse. Laurel Lance's Earth-2 counterpart, a villain called Black Siren, will arrive in the May 17th episode of "The Flash" -- and she'll be in charge of Zoom's metahuman army. The CW has released the synopsis for the episode, which is titled "Invincible," and you can check it out below.

THE FLASH "Invincible" - (8:00-9:00 p.m. ET) (Content Rating TBD) (HDTV)
KATIE CASSIDY GUEST STARS AS BLACK SIREN - After Zoom (guest star Teddy Sears) unleashes an army of Earth-2 meta-humans on Central City, Barry (Grant Gustin) is shaken when he sees their leader is the Black Canary's Earth-2 doppelganger, the Black Siren (guest star Katie Cassidy).  Meanwhile, Wally (Keiynan Lonsdale) takes to the streets to help The Flash stop the meta-humans, which worries Joe (Jesse L. Martin).  Iris (Candice Patton) and Henry (guest star John Wesley Shipp) are concerned about Barry taking on Zoom.  Jesse Warn directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Brooke Roberts & David Kob (#222).  Original airdate 5/17/2016.

"The Flash" airs on the CW on Tuesdays at 8 PM ET.