The title credits are in black and white and only the pen ink is blood red. Music is more dramatic. This is 15th time the title credits were altered to fit the episode theme.
When Castle, Hailey and Alexis go into the safe, they find the date 5-10-16 on the wall which Castle says is "the end of days". The episode was aired 1 day before, on May 9, 2016.
Wormwood, the false name used by Gabriel Shaw's visitor, is the name of one of the demons in The Screwtape Letters.