Legends of Tomorrow star Caity Lotz will return to Arrow as Sara Lance in the season 6 finale. Sara's role in the story has yet to be revealed, but there are a number of plot-lines on Arrow that could explain her return to Star City.
Before joining the crew of the Waverider on The CW's Legends of Tomorrow, Sara Lance was the original Black Canary and a recurring character during season 2 of Arrow. The character was murdered in the beginning of season 3 only to be resurrected in season 4. As the White Canary, Sara Lance was recruited by Time Master Rip Hunter (Arthur Darvill) to save the world from Vandal Savage in Legends of Tomorrow. The character has since gone on to become the captain of the Waverider.
Collider reports that Sara Lance will appear in the season 6 finale of Arrow, which will air on May 17, over a month after Legends of Tomorrow concludes its third season with "The Good, The Bad, and the Cuddly". The Legends season 3 finale will feature the final showdown with Mallus with guest-appearances from John Constantine (Matt Ryan) and Jonah Hex (Jonathon Schaech), along with the return of Jax (Franz Drameh).
It's unclear what will bring the White Canary back to Star City for the Arrow finale, but fans are likely to speculate that her presence in the city is tied to the character of Black Siren (Katie Cassidy). Black Siren is an alternate universe version of Sara's deceased sister, Laurel. In the last episode of Arrow, Black Siren was beginning to earn Oliver's trust, but was revealed to be secretly working for Diaz (Kirk Acevedo). Black Siren has also revealed herself to the public as the real Laurel Lance. If this news were to reach the Waverider, it could easily bring Sara back to Star City.
There's another storyline brewing that could potentially have ties to Sara. In tonight's episode, "The Thanatos Guild", Team Arrow will be clashing with a rogue group of League of Assassins members. Oliver and the others will have to work with Nyssa (Katrina Law) to stop them. Fans will recall that Nyssa is Sara's ex-girlfriend. If this storyline lasts longer than just a few episodes, it's possible that Sara's return will be linked to Nyssa and the League of Assassins.
Arrow continues tonight with 'The Thanatos Guild' at 9pm ET on The CW.