Diophantine Pseudonym

S1 E6: After being captured by Mal'akh, Langdon is finally reunited with his mentor, Peter. Katherine tracks down Mal'akh's lair by watching Peter's hostage tape again and again. In the process of escaping from Mal'akh with Langdon, Peter glimpses something familiar in his captor's eyes and realizes Mal'akh has been Zachary this whole time. He shares the information with a shocked Sato. To avoid being caught, Mal'akh jumps off a cliff to his presumed death. But in truth, he's alive and has been arrested by the CIA.