WARNING: The following contains spoilers up to Season 4, Episode 10 of Attack on Titan, "A Sound Argument," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
Eren Jaeger has been a primary protagonist since the beginning of Attack on Titan, with his world falling to pieces when his mother, Carla Jaeger, was eaten by a Titan. While he remembered what happened to his mother, his memories about what happened to his father, Grisha Jaeger, were blurry, and things only got more confusing for Eren when he learned he was a Titan shifter back in Season 1, Episode 8, "Hearing the Heartbeat - The Battle for Trost (4)."
This was the first time audiences, as well as the main cast born within the walls, learned that some people could turn into Titan shifters. Along with that, Eren seemed to be a special shifter. Where Reiner Braun, Bertholdt Hoover and Annie Leonhart inherited the powers of a single shifter, Eren had the powers of two shifters until Season 4, where he inherited another Titan's abilities. Looking back at Attack on Titan, here is how Eren became one of the most dangerous shifters to date.
The Attack Titan
Eren would learn he was the Attack Titan in Season 1 at 15-years-old; however, he was not aware that he inherited the Attack Titan from his father five years earlier. In fact, the Eldians of Paradis, outside of the royal family, did not know about shifters until Eren turned into the Attack Titan. While he simultaneously inherited the Attack and Founding Titans, the characters assumed he was just the Attack Titan until Season 2.
Prior to Eren becoming the Attack Titan, his father, an Eldian from Marley, was this shifter, inheriting its powers from Eren Kruger, an Eldian spy who infiltrated Marley's ranks. After turning nearly every captured Eldian Restorationist into a mindless Titan and releasing them on Paradis, Kruger spared Grisha and revealed who he really was.
He also disclosed that his time was nearly up as a shifter because of the Curse of Ymir. Shifters only live for 13 years, so Kruger needed someone to inherit the Attack Titan, reclaim the Founding Titan and free Eldians for good, which Grisha agreed to. From there, he would live within the walls, start a new family and eventually confront the Founding Titan after Wall Maria was breached in 845.
The Founding Titan
Grisha found the Reiss family -- the royal family -- hidden in the Underground Chapel. Unaware that Carla was already dead, Grisha begged them to use the Founding Titan to kill the mindless Titans in order to save his family; however, Frieda Reiss, the Founding Titan at the time, was overcome by King Fritz's decision to renounce war, which meant she could not retaliate against the invading Titans.
Instead, her Titan fought Grisha's Titan, with the latter winning and consuming Frieda, thus inheriting the Founding Titan, which until then had only been passed down to members of the royal family. However, returning from his mission, Grisha learned it was too late, and Carla died. With his time also running out as a shifter, Grisha instructed Eren to avenge his mother before taking him into the woods. There, he injected his ten-year-old son with Titan serum, with the child soon turning into a mindless Titan and eating Grisha.
When he'd awaken, Eren would not know what happened to his father, and it wouldn't be until Season 2, Episode 12, "Scream," that audiences, as well as Bertholdt and Reiner, would learn he was the Founding Titan, a.k.a. the Coordinate Titan. Throughout Season 3, Eren would get more conclusive answers about how he became the Attack Titan and Founding Titan, learning the truth about his father and the outside world.
The War Hammer Titan
While Eren inherited the Attack and Founding Titans from his father at the same time, it wouldn't be until Season 4, Episode 7, "Assault," that he would inherit the War Hammer Titan. Similar to the Founding Titan, the War Hammer Titan was kept within a specific family, being passed down to members of the Tyburs, who were the only Eldians the outside world respected. At the time of Season 4, Willy Tybur's sister was the War Hammer Titan.
The Tyburs visited Liberio, a city in Marley, in 854, with Willy planning to redeem Eldia's reputation by revealing the truth about King Fritz and proclaiming that Eren's the true threat to peace because he stole the Founding Titan from the Reiss family. He also declared war against Eren, who subsequently transformed into his Titan and ate Willy.
The War Hammer Titan was quick to jump into action, but Eren eventually grabbed hold of its user, who encapsulated herself in a nearly indestructible crystal. Eren used the Jaw Titan to break the crystal, killing Lady Tybur and inheriting the War Hammer Titan after swallowing her blood. Audiences have yet to see Eren use the War Hammer Titan's abilities; however, he threatened to use them in Season 4, Episode 10, "A Sound Agreement." This act would also make Eren all the more dangerous as he is now in possession of three powerful Titans.