Heroes Join Forces is the name of the second annual shared multiverse crossover event between and . It also served as a set-up for by introducing characters who would be featured in that series. The crossover aired over two nights from December 1 to December 2, 2015.
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 Arrow 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 shows up and flies off with Kendra.
Oliver and Barry take Kendra and Carter Hall to a remote location to keep them hidden from Vandal 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.
- Paul Blackthorne is the only series regular who didn't appear in this crossover.
- This is the first crossover to include Katie Cassidy.
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