Throughout numerous posts made by the Arrow cast and crew on social media, several fan favorite characters are returning for the 100th episode, which is the third part of the major crossover event. This includes Katie Cassidy (it's unknown if it's Black Canary or Black Siren), Robert Queen and Deathstroke (not sure if it's Manu Bennett under the helmet). Marc Guggenheim also said on his Twitter that it is NOT flashpoint of flashbacks. The only thing known is that those three characters are returning and that there may be more. Let me know what your theories are for their return.