The Doctor is told to put down all his weapons and he places his spoon on the table. In Robot of Sherwood (2014) he uses a spoon as a weapon when fighting Robin Hood.
The four knocks the Doctor hears are the same as the sound of drums the Master heard in the third series, and a reference to the four knocks that Wilf inadvertently used to kill the Tenth Doctor in The End of Time: Part Two (2010).
Steven Moffat talked about the final two-parter of season 9 with Doctor Who Magazine and said "Episode 11 pushes the Doctor to the brink of madness, and Episode 12 is what happens next. If the Doctor has lost his moral compass, if he's being selfish, if you really, really hacked him off, if you really got him angry and gave him nothing to fight for.. what would you end up with?
That's the 'hellbent' of the title. An angry, off-the-rails Doctor."