The anime, Attack On Titan, is likely one of the more intense and goriest available. Humans live inside the walls for what they think is protection, but when the Titans reappear and start breaking through, Eren Yeager and his friends feel they need to intervene.
Eren, Armin, and Mikasa enlist in the Armed Forces of Eldia, and instead of going to the innermost wall to protect the king, they decide to stay and face the Titans head-on. This show starts off letting the audience think it will be about becoming the best version of oneself for the protection of the greater good; While that is true, Eren is out for revenge and faces some of the saddest hardships, someone could ever imagine handling.
His Mom Was Eaten By A Titan
Eren's seemingly peaceful life is met with chaos almost immediately when the Titans get through the wall and his mom, Carla, is crushed in rubble and he cannot pull her out.
His mom then gets eaten by a Titan right in front of him shortly after he has to run to avoid getting eaten as well.
He Is A Titan
When Eren became a Titan, he became a monster in the eyes of the Armed Forces of Eldia. Eren is eaten by a Titan, escaped as a Titan, and later emerged as a human.
He is faced with the confusion of what had just happened to him and why. And before he could truly think about it, the military surrounded him and his friends, and he has to come up with a plan before they all lose their lives.
His Dad's Former Wife Is A Titan
Before Eren's father, Grisha Yeager, had a family in Eldia, he had one in Marley. His former wife, Dina, was turned into a titan by Marley's military and sent to Paradis Island. The "Smiling Titan," who killed Eren's mom turns out to be Dina.
Eren sees Dina in Titan form again when he is fighting the Armoured Titan. Dina then kills Hannes, the guard Eren befriended, right in front of him.
Guilt Of Losing His Squad
Eren blames himself when Gunther, Eld, Petra, and Oluo die at the hands of the Female Titan in the forest battle of season one.
Not only does the audience see him deeply regretting not turning into a Titan earlier in the fight, but they see him losing himself as he falls deeper into his need to kill this female Titan.
He Ate His Dad
In order to stop the attack on Wall Maria, Grisha attempts to get the Titan coordinator to call it off, and when that fails he transforms into a Titan and eats her.
After Grisha injects Eren with Titan serum, Eren eats his father but doesn't remember this until years later. Eren doesn't know his family history yet and is subjected to Titan life without any semblance of what it is like.
Betrayal
Eren learns throughout the series that 4 of his military comrades are betraying him and are actually Titan-Shifters.
Annie Leonhart, Ymir, Reiner Braun, and Bertholdt Hoover, are all eventually revealed as shifters either by force or through a scenario in which they had to shift with witnesses.
The Eldians Are Turning Into Titans
Eren learns that his race, the Eldians, are able to turn into Titans and the country across the ocean is the one creating Titan serum. Titan serum is created from the spinal fluid of a Titan and is easily injected.
Once the Marley's turn Eldians into Titans, they send them to destroy the walls and cause chaos on Paradis. It was a startling revelation for everyone within the walls, but to Eren it meant that there was more war to come.
His Witness Of The Future
By the end of Season 3, Eren thinks his dreams are about to be realized on the outside of the walls but he has seen the future of his friends.
When he kisses Historia's hand, Eren sees the interaction between Grisha and the Reiss family and witnesses that Eren himself, will bring death to everyone.
Hizuru's Refusal
In the most recent season, Eren attends a meeting with Eldia and a representative from the nation, Hizuru. Hizuru initially agrees to help the Eldians in becoming a stronger nation and by helping them with trading, but later goes back on the deal. The event essentially means Eldia does not have a clear way to enact peace with the other nations. Eren begins to act differently after realizing he will have to pass on his Titan soon and he suddenly starts considering Zeke's plan to start the "rumbling."
He Kills Innocent People
In order to free himself and his island, Eren ends up just like Reiner and kills innocent people. His recent discovery that being a Titan is living a nightmare is similar to Armin's realization, "In order to defeat monsters, you have to be willing to throw away your humanity."
During the "Declaration of War" episode, Eren realizes he cannot save everything, he turns Titan and wreaks havoc in Liberio, an internment zone, killing innocent people without cause because his anger gets the better of him.