Episode 21 of Season 3 (Episode 58 overall) of Attack on Titan The Attack Titan Le Titan Assaillant
The episode introduces the concept of "paths" that connect all "Subjects of Ymir" through space and time, all leading to the "Coordinate" (the Founding Titan). A Message from the Future: Shingeki No Kyojin -Attack On Titan- S3 21 VF