Actress Katie Cassidy is set to appear in an episode of "The Flash" season 2 but will not be reprising her role as Black Canary. Instead, according to a report from TV Line, she will be portraying her Earth-2 counterpart known as the Black Siren. 

Black Siren is taken directly from the DC Comics canon, where she was an original member of the Justice Society and later the mother of Dinah Laurel Lance, who became the comic books' iteration of Black Canary.

This origin was dropped on "Arrow" and "The Flash" as Dinah (Alex Kingston) is not portrayed as a superhero, instead handing the story to Laurel's sister Sara (Caity Lotz). It now appears that their Earth-2 counterpart will still keep Laurel as the original Black Siren and still keep Dinah as a civilian.

However, as stated in a report from Screen Rant, it is likely that while Black Siren will be similar to Black Canary, the Earth-2 version will be darker and most likely more violent. This has been the case with many characters from Earth-2. How Earth-2 and Black Siren will be introduced into "The Flash," however, remains a mystery.

Barry Allen aka The Flash (Grant Gustin) did manage to close all the breaches connecting Earth-1 to Earth-2, but in recent episodes, he has managed to show the capability of traveling through time, which indicates he may later gain a new method of traveling through different parallel universes, giving him access once again to Earth-2.

The report also points out that Cassidy's appearance on "The Flash" indicates she might be the character that dies in "Arrow," the character that Oliver Queen (Stephen Amell) is shown grieving for at a cemetery. However, this has yet to be confirmed and it is likely yet another tease for the finale of both shows.

Black Siren will appear in the penultimate episode of "The Flash," airing on May 17.