CW has debuted a new clip for the upcoming first part of the which airs tonight, December 1, 2015.
The first half of "The Flash" crossover, which is titled "Legends of Today," is officially described as;
"In its second major crossover event with “Arrow,” Vandal Savage (guest star Casper Crump) arrives in Central City and sets his sights on Kendra Saunders (guest star Ciara Renée).
After Vandal attacks Kendra and Cisco, they turn to Barry for help. Realizing how dangerous Vandal is, Barry takes Kendra to Star City and asks Oliver and team to hide her until he can figure out how to stop Vandal.
However, things quickly go from bad to worse when a man with wings AKA Hawkman (guest star Falk Hentschel) shows up and flies off with Kendra. Meanwhile, Harrison develops a serum to make Barry run faster and asks Jay (guest star Teddy Sears) to test it out.”
The "Arrow" half of the crossover is titled “Legends of Yesterday” and is officially described as;
“Oliver and Barry Allen take Kendra Saunders and Carter Hall to a remote location to keep them hidden from Vandal Savage while they figure out how to defeat him.
Malcolm arranges a meeting between Vandal, Green Arrow and The Flash that doesn’t go as planned. Meanwhile, Felicity, Thea, Diggle and Laurel work with Team Flash to come up with a weapon powerful enough to destroy Vandal Savage.”
The crossover episodes will feature nine superheroes, and launch the anticipated spinoff of both sows, titled, undefined