Finally, the much awaited crossover episode between "The Flash" and "Arrow" is happening on Wednesday and the first part will be "The Flash" eighth episode this season.

In this two-part special, the team will battle the immortal, Vandal Savage, who will also be the main antagonist in the upcoming TV series that is also a cross-over of "The Flash" and "Arrow," "Legends of Tomorrow."

Based on the new trailer, it looks like Savage is after Hawkman and Hawkgirl but his intentions are yet to be revealed. Both winged fighters will ask for the help ofCentral City's defenders, Team Flash and Star City's heroes, Team Arrow. Here's the trailer:

Also, here's the synopsis for the upcoming episode courtesy of "The Flash" season 2 episode guide wikia:

"Vandal Savage arrives in Central City and sets his sights on Kendra Saunders. 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, also known as Hawk-Man shows up and flies off with Kendra. Meanwhile, Harrison develops a serum to make Barry run faster and asks Jay to test it out."

Meanwhile, "The Flash" episode 9 marks the debut of Keiynan Lonsdale, who will portray the role of Wally West, Joe's long lost son. Here's the summary from Comic Book Resources.

"MARK HAMILL, WENTWORTH MILLER AND LIAM MCINTYRE RETURN; KEIYNAN LONSDALE JOINS THE CAST AS WALLY WEST - When Mark Mardon AKA The Weather Wizard (guest star Liam McIntyre) returns to break Leonard Snart AKA Captain Cold (guest star Wentworth Miller) and James Jesse AKA The Trickster (guest star Mark Hamill) out of Iron Heights, Barry (Grant Gustin) must stop these rogues from taking over Central City during Christmas." 

"Meanwhile, Joe (Jesse L. Martin) and Iris (Candice Patton) meet Wally West (Keiynan Lonsdale).  Kevin Tancharoen directed the episode written by Andrew Kreisberg (#209)."