Since its debut in April 2013, "Attack On Titan"  has become nothing short of a major success. Based on the manga by Hajime Isayama, the story features an apocalyptic world in which towering, zombie-like giants known as Titans roam the land, forcing humanity to retreat behind three massive walls as they seek to find ways to destroy them.

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Since the end of season one, fans of the anime have been clamoring for a second season, and their prayers were finally answered with an official trailer and debut date of April 1."Attack On Titan" is a dark and gruesome series, and its anime has managed to pull in quite a number of fans. So, to help usher in the continuing fight for humanity, we've decided to take a look back at some of the more disturbing and shocking moments from the first season.

WARNING: MAJOR SPOILERS FROM SEASON ONE OF "ATTACK ON TITAN."

15 SMILING TITAN

The moment that people watching this series for the first time knew that this would be an entirely different show the moment the "Smiling Titan" appeared. A female titan with short hair and a creepy smile that could even creep out The Joker, this particular titan is responsible for the death of Eren's mother, eating her as she was immobilized and unable to move.

The Smiling Titan was a defining moment in Eren's path, as it was at this moment, coupled with the fall of Wall Maria and the invasion of his home that he dedicated his life to killing every last Titan. To that end, he joined the Scouting Regiment and graduated near the top of his class. Regardless of this, the Smiling Titan remains one of the more iconic and creepy visages to be seen within the series, and it only took seeing her in one episode to cause a massive ripple effect throughout the viewers watching this series as she chomped down on Eren's mother.

14 INVASION OF SHIGANSHINA DISTRICT

The death of Eren's mother was only the tip of the iceberg that kicked off "Attack On Titan." The appearance of the Colossal Titan, which towers above even the great walls of humanity, was a disturbing realization that made everyone realize just the kind of insurmountable situation humanity found themselves in as it created a massive hole and flooded Eren's home district with Titans.

The appearance of the Colossal Titan was disturbing for a myriad of reasons. His towering size, coupled with the fact that he doesn't have any actual skin or hair like other Titans, makes him look like a larger-than-life human anatomy model. His random appearance from seemingly out of nowhere is a mystery unto itself, providing yet another jolt of fear that this bare Titan has the ability to just as easily bring down the other two walls in the same manner as it did Maria. Heading into season two, it looks as though we'll be seeing more of the Colossal Titan and perhaps unraveling its bigger mysteries.

13 INVASION OF TROST DISTRICT

This is the point where it seems, at first, that "Attack On Titan" was going to resemble something hopeful, as Eren and his friends recently completed their military training and decided to join the Scouting Regiment instead of a more cushy Military Police position. The first day stationed at the Wall results in the appearance of a familiar friend: the Colossal Titan.

In a similar fashion with Wall Maria, the Colossal Titan makes a gaping hole and mysteriously vanishes, leaving Titan fodder to rampage into the District. The results are much the same, bringing about yet more disturbing images of these Titans ravaging veterans within the military and rookie cadets of Eren's graduating class on what was supposed to just be another routine day that, despite the threat of the Titans, remained hopeful that they could be the class that finally earns a victory for humanity. The invasion of Trost is disturbing in its own similarity to Shiganshina, and even an older Eren, despite his newfound and improved skills with Omni-Directional Gear, realizes just how grand the war against the Titans is and the powerlessness that still remains within him to stop them.

12 FEMALE TITAN APPEARS

In their first scouting mission outside the walls, the Survey Corps ventures outside the walls in order to somehow find any connection or source of the Titan threat. They've had no luck up to this point, though this time, they have the recently-revealed Titan Shifter Eren as a trump card and the recent victory in Trost to boost morale for the unit. Unfortunately, a disturbing new development appeared: the Female Titan.

Similar to the Colossal Titan in appearance when it comes to lacking skin, the Female Titan first appeared before the Survey Corps, specifically among Armin's group. Showing the kind of speed and intelligence among any Titan, it decimated the unit he was assigned to, easily catching up to them on horseback and moving unlike any average Titan they'd seen before. Perhaps the most disturbing aspect was that, for some reason, the Female Titan decided to spare Armin's life despite taking everyone else's. The discovery of this Titan, which seemed eerily similar to both the Armored and Colossal Titans, unveiled more questions about how perhaps these human-eating Titans weren't even the biggest threat humanity has to deal with.

11 LEVI BEATS UP EREN IN TRIAL

Upon being revealed to be a Titan Shifter, Eren's life hung in the balance. Despite saving Mikasa and his deeds during the battle in Trost, Eren's fate still hung in the balance thanks to the breach in the wall and the human's fear of this interesting phenomenon. So, he is taken into military court where he is fated to be handed over to one of two places: the Survey Corps or the Military Police.

If Eren is handed over to the Military Police, it means certain death. On the other hand, the Survey Corps sees Eren's use as a potential trump card against the Titans. Fearing his transforming into a Titan, many are afraid to go near him. However, Levi Ackerman takes matters into his own hands, brutally beating Eren in front of the spectators and not holding back any of his punches or kicks against the rookie military member. While Levi's actions ultimately saved his life, it demonstrates the extent of the turmoil within the dwindling ranks of humanity within the walls and the extremes that had to be brought about for Eren to survive.

10 ERWIN TRAPS FEMALE TITAN

Upon hearing news about this mysterious Female Titan, Erwin and the Survey Corps decide to dedicate their mission to capturing this mysterious titan and unveil its identity. Doing so required luring it into a trap, which, given its apparent intelligence, wasn't going to be an easy task. Needless to say, it took a lot of lives in the process.

Eventually, they managed to draw the Female Titan into their dastardly trap: a large combination of grappling hooks that shot into its body from top to bottom to immobilize it while figuring out the identity of the Titan Shifter. But it's all in vain, as the Female Titan ushers a mysterious cry that draws nearby Titans to her, which ravage her body and prevent the humans from unveiling the secrets within it. Seeing the other Titans ravage its body on the command from its cry conveys yet another disturbing moment, as it is one of the few instances we see Titans eat another Titan body, and seemingly on command at that. If there's one thing humanity doesn't need, it's a powerful Titan commanding all the smaller, human-eating ones.

9 PARTIALLY TRANSFORMED TITAN FORM

In the midst of fear, humans do some crazy things. In the midst of getting out of Trost, Eren mysteriously emerges from the body of a large Titan that managed to save Mikasa's life. Getting beyond the Trost District behind the wall, Eren, Mikasa and Armin suddenly find themselves with cannons and guns pointed at them from the military thanks to Eren.

Unable to reason with the fearful commander's fear of Eren, he is quickly forced to take quick action to protect Armin and Mikasa, who are the only close friends Eren has left. Thinking on his feet, he instinctively activates his newfound Titan power. The result: a partial transformation of a Titan's skeletal torso that manages to protect all of them from the gun and cannon fire they found themselves under in addition to the now Titan-infested Trost district. This ominous partial Titan form serves to confirm Eren's mysterious newfound ability while displaying a fearful image of what this power represents and the new unknown position humanity finds themselves in with this discovery.

8 HANGE'S TITAN OBSESSION

It's no secret that humans in "Attack On Titan" are quite afraid of, well, Titans. Gigantic creatures who strive to solely feed on your species tends to do that. However, Hange doesn't quite fit that mold of the normal Titan-fearing humans cowering behind their massive walls.

Another skilled member of the Survey Corps, Hange is obsessed with learning about the Titans and their mysterious origins. She is enamored with them to the point that she named two Titans captured by the Survey Corps Sawney and Beane, who are subject to the various tests and whims Hange puts them through to learn more about their abilities and anatomy. Her eccentric personality stands out among even the Survey Corps, and she is perhaps the only one ever interested in doing more to a Titan that splitting the nape of its neck as quickly as possible. Needless to say, when her Titan test subjects were both mysteriously killed, it didn't go over well for her. If there's ever anything that's creepier than any rampaging Titan, it's Hange's quirky, energetic obsession over them.

7 EREN AND MIKASA MURDER TRADERS

Among humans, the world of "Attack On Titan" is a dangerous one where humans still must be wary of one another, as brutality still remains between them despite being on the edge of extinction. One such example includes the backstory of Mikasa's background, in which her strong attachment towards Eren is explained.

Having her home invaded by human slavers, Mikasa's parents were killed and she was set to be kidnapped by them. However, a young Eren returns to the house and urges Mikasa to realize that, in their bleak world, they have only two choices: fight or die. Spurred on by his words, a young Mikasa gains the courage she needs to kill one of the slave traders while Eren kills the remaining two after catching them by surprise. Because of her parents' deaths, Mikasa goes on to live with Eren and his family and she becomes forever dedicated to him. To have these young children commit these murders against such criminals shows the kind of dark, twisted world the characters find themselves in while showing the biggest threats come from more than the Titans.

6 EREN'S DEATH

A once hopeful campaign to begin the fight for humanity anew came to a screeching halt on the first day of Eren's assignment when the Colossal Titan shows up and punches yet another gaping hole in the wall into the Trost district. Things take an even sharper turn when, in an attempt to save Armin from getting eaten, Eren himself is eaten instead and loses an arm in the process.

When Eren wakes up, he's in the Titan's stomach, surrounded by the bits and pieces of his comrades. In one of the most gruesome displays in the series, seeing the Titan's stomach is a disturbing experience, as Eren is nearly up to his next in blood and stomach acids. Set to be digested like every other poor human that got eaten, it turns the tables on everything. All the training, hard work and resolve to kill these Titans is meaningless if you get caught in their grasp. For all intended purposes, Eren's journey ended the moment he was swallowed in an attempt to save his friend. Witnessing the belly of a Titan is an experience viewers only need to see once to get the message.

5 MYSTERY TITAN APPEARS

Through most of season one of "Attack On Titan," Mikasa is a constant embodiment of fearlessness, always seeking to protect the only family she has left in Eren and Armin. Never one for many words, she was skilled enough to graduate at the top of their class. Even so, we all have our weaknesses, and when Armin delivers news to her of Eren's apparent death, she quickly becomes reckless, using up all the gas of other Omni-Directional Maneuver Gear and finding herself caught defenseless between two Titans.

Suddenly, Mikasa finds herself shielded by the appearance of some mysterious Titan who easily dispatches the others on the ground. Not only that, it also appears to be more intelligent than other average Titans introduced and seemed to ignore the other humans. To witness this mysterious Titan savagely kill others of its own kind is nothing short of a disturbing phenomenon, as the creatures never openly attack one another, let alone defend the humans they try so hard to consume. It also added to the greater question of whether new types of Titans are beginning to suddenly emerge.

4 DEATH OF LEVI SQUAD

Out of all those in the Survey Corps, Levi's squad was considered the best of the best. With Gunther, Oruo, Eld and Petra, each of them is famous within the military for their ability to slay Titans single-handedly and their devout loyalty to their commander Levi. They were the best fighters humanity had outside of the wall.

Suddenly, all of that is taken away as they die defending Eren from the Female Titan, who appears to pursue the former for reasons unknown. The gruesome manner in which they die is due to Eren's belief in their skills to take down this new threat. Unfortunately, they are unable to account for the Female Titan's speed and hardening ability with each of them falling one by one. To see such veterans fall continually reawakens the continual message that no one is safe from these Titans, and even skilled members like these can fall. It's only run in deeper when Petra's parents run up to Levi looking for her once they return within the walls. Their deaths are yet another grim and disturbing reminder that it takes more than pure belief to survive for very long in this world.

3 SCOUT REGIMENT TOSSES BODIES AWAY

While on their way back home with the corpses and remains of their comrades amidst their failed mission, the Survey Corps find themselves being pursued by Titans. Left with no choice if they themselves want to survive, they are forced to toss their remains away to shake off the Titans in pursuit throwing them off the cart they're being transported on to also increase their speed.

Seeing the Survey Corps have to sacrifice these bodies just to make it back to the wall signifies the kind of hopeless situation they find themselves in. The gruesome act of the blood-soaked wrapped bodies being tossed from the cart from beleaguered Corps members is enough to make anyone shudder. To make matters worse, their failure is met with scorn from the townspeople and they aren't very well received from the higher-ups in the military upon their return. The scene serves to illustrate the hard job the Survey Corps has when journeying outside of the walls and paints a grim picture of the reality they have to deal with every day, which is much different than the idealistic visage Eren painted it with at the beginning of the series.

2 FEMALE TITAN IDENTITY REVEALED

The Female Titan caused a ton of grief and took a lot of lives within the Survey Corps. After returning from their mission, Armin reveals that he has unveiled its identity: it was, in fact, Annie Leonhart, one of the most skilled members of their entire class.

All through the series, Annie has been among the most skilled and trustworthy of the new cadets. Skilled in hand-to-hand combat, she taught Eren a thing or two during training and easily overpowered him on a regular basis. To see her revealed as a Titan Shifter and her reaction upon being found out by none other than timid Armin brings about a strange personality shift from the normally stoic Annie, who breaks out laughing in a rather strange manner before initiating her transformation into the Female Titan and starting a final showdown against Eren in Stohess. Having such a talented member of the Survey Corps turn traitor brings another disturbing thought into question: just how many people like Eren and Annie might be with them within the walls?

1 TITAN WITHIN THE WALL

As "Attack on Titan" wore on, a number of mysteries became apparent. Why does Eren have this Titan-shifting power? Where did it come from? How many people besides he and Annie are with them inside the walls? What are the deeper mysteries surrounding the Colossal and Armored Titans?

As engaging as those mysteries are, none are perhaps creepier than the final moments of the season one finale, in which a piece of Wall Sina breaks and shows a shocking revelation: a Titan inside of the actual wall itself. Based on its vacant looks, the Titan appears to be inactive, given that they need exposure to sunlight and seem to draw their energy from it, as was seen from Sawney and Beane when they were taken away from it. Their bodies also dissolve once they die, which further signifies that this exposed Titan is very much alive. Nonetheless, this opens the door to a whole new array of questions as season two begins. From this, it's easy to see that Annie's mysterious circumstances and origins are only the tip of the iceberg and that the remainder of the Survey Corps' hardest fight has yet to come.

What were some of your favorite or disturbing moments from season one of "Attack on Titan?" Let us know in the comments!"