The CW has just announced that Katie Cassidy will return to Arrow as the Black Siren for the final two episodes of the show's currently-ongoing fifth season, then continue in the same role (Black Siren) as a series regular for season 6.
The character -- Earth-2's version of Cassidy's character Laurel Lance -- debuted on The Flash last season and then came to Arrow this season, where she worked with season 5 big bad Prometheus. During that storyline, Black Siren pretended to be Black Canary, back from the dead, but was quickly revealed.
"One of the things that most excites us about Arrow is that we go where the story takes us," said Arrow executive producer Marc Guggenheim. "Last year, the story took us to the tragic death of Laurel Lance. This year, our midseason finale reintroduced us to Laurel’s doppelgänger, Black Siren. We were so taken by Katie’s fearless interpretation of this character that we knew her story was not yet finished. We couldn’t be more thrilled to welcome Katie back to Arrow as Black Siren, though as her appearances on Arrow and DC’s Legends of Tomorrow this season show, she never really left the family."
Cassidy had been a series regular on Arrow from the get-go, first as Oliver's primary love interest in the first season and later as a supporting character in the District Attorney's office by day and on Team Arrow by night. When her character died during a prison riot in season 4, fans expected her to be resurrected (like her sister Sara, played by Caity Lotz on DC's Legends of Tomorrow) was, but it wasn't to be.
Cassidy had been announced along with DC's Legends of Tomorrow star Wentworth Miller, who played Captain Cold in the show's first season (before that character, too, was killed), as kind of shared series regulars who would be on call between the various shows of The CW's DC Universe at the start of this season. Fans expected more from the pair, who between them have appeared in about a half-dozen episodes so far, but it seems likely that being tagged as a series regular on Arrow is likely a slightly different move.
Arrow airs Wednesday nights at 8 p.m. ET/PT on The CW.