The CW has released an official description with spoilers for the December 7 episode of Arrow.
The episode is called “Irreconcilable Differences” and the description can be found below.
BLACK SIREN KIDNAPS LANCE— Quentin Lance (Paul Blackthorne) is kidnapped by Black Siren (Katie Cassidy) and Cayden James (guest star Michael Emerson), who are looking to trade Lance’s life for a weapon of mass destruction. Oliver (Stephen Amell) and Team Green Arrow must figure out a way to save Lance while still keeping the city safe. Laura Belsey directed the episode written by Beth Schwartz & Sarah Tarkoff (#609). Original airdate 12/7/2017.