Katie Cassidy Guest Stars on Tonight's 'The Flash'!

Katie Cassidy looks badass as Black Siren in this still from tonight’s episode of The Flash.

In “Invincible,” after Zoom (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.

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 (John Wesley Shipp) are concerned about Barry taking on Zoom.

The Flash airs TONIGHT, May 17 @ 8PM on The CW.

15+ stills inside…