: This detailed article from Pop Culture References explores how the episode serves as a major clue to Reddington's true identity, showing Kathryn's life as Liz's nanny and her initial bond with Katarina.
(Season 4, Episode 17 / Often cited as S4E8 in specific contexts)
(Season 7, Episode 22 / Sometimes confused with S4) [S4E8] Requiem
While officially the 17th episode of Season 4, "Requiem" is a pivotal origin story often grouped with mid-season highlights. It focuses on the backstory of (Kathryn Nemec) and her history with Katarina Rostova and Masha (Liz).
(Video Game): While not a TV episode, this recent release has generated significant discussion regarding its ending and the character Grace. : This detailed article from Pop Culture References
: For a broader look at the secrets revealed during this era of the show, the Raymond Reddington Wikipedia page summarizes the "imposter" theory and the role Kathryn played in hiding the truth.
Though it is the Season 7 finale, "Requiem" is one of the most famous episodes in the series, featuring the abduction of Fox Mulder. (Video Game): While not a TV episode, this
: Titled "Gold, Glory & Custer: Requiem Part II," this episode is praised by history buffs for its authentic use of Lakota dialogue and historical figures like Chief Touch the Clouds.