The Marineford Arc is probably by far the biggest turn of events in One Piece history as it features the first two deaths of major characters in the current story line In particular, the death of the Emperor Whitebeard, along with the resignation of two Warlords of the Sea (Jinbe and Teach) and one revoked (Moria), drastically altering the balance of the Three Great Powers. Ace's father is revealed to be Gol D. Roger and more details come to light about how Ace became a member of the Whitebeard Pirates. It is revealed to the world that Luffy is Dragon's son and Garp's grandson. Ace is killed by Admiral Akainu, the third Admiral to be introduced, which causes Luffy to fall into a comauntil he recovers in the Post-War Arc. Before his death, Ace mentioned Sabo, who is introduced in the flashbacks Luffy has after he awakes, and later plays a major role in the story. As revealed in the Dressrosa Arc, Ace's death would have an effect on Sabo who unknown to both Ace and Luffy at the time was still alive. As a result, this helps Sabo recover his memories after being stricken with amnesia for ten years. It is also the reason why he is so determined to protect Luffy so he would not have to lose another "sworn brother". After Ace dies, Whitebeard predicts that someone will "come to inherit his will." This prediction later comes true as Sabo eats Ace's Devil Fruitafter he dies and specifically voices his desire to inherit the late pirate's will, mirroring Whitebeard's prediction exactly. Marshall D. Teach is revealed to have recruited five new members into his crew from Level 6 of Impel Down: Sanjuan Wolf, Catarina Devon, Vasco Shot, Avalo Pizarro and Shiryu. The entire Blackbeard crew (except Sanjuan Wolf) attacks and kills Whitebeard. In his dying breath, Whitebeard confirms to the world the existence of One Piece and foreshadows a huge war once it is found. He even gives more information about the mysterious Will of D. His death leads to the loss of protection over many islands, including Fish-Man Island. Blackbeard steals the Gura Gura no Mi's powers from Whitebeard's dead body by unknown means, thus becoming the first man ever to obtain the power of two Devil Fruits. In the Fish-Man Island Arc, it is said that the Blackbeard Pirates are hunting other Devil Fruit users in order to steal their powers. In the Wanoi Country Arc, it is revealed that Shiryu also got a former Devil Fruit user's powers after the latter was killed, the Thriller Bark Pirates' member Absalom. Jinbe renounces his Warlord status and in the next arc after he parts ways wth Luffy, he makes a promise that they will meet 2 years later in Fish-Man Island. It is insinuated that Ivankov had a past relationship with Bartholomew Kuma. Later, it is revealed that Kuma is a Revolutionary. Koby's Haki is activated for the first time in a way similar to Mantra. In the Post-War Arc, it was revealed to be called Kenbunshoku Haki. Ace's death would cause his Devil Fruit, the Mera Mera no Mi, to resurface later and be in the possession of Donquixote Doflamingo in the Dressrosa Arc as a means to lure Luffy to him. Because of Blackbeard murdering Whitebeard, the remnants of the Whitebeard Pirates would later fight the Blackbeard Pirates one year after the war at Marineford. The Whitebeard Pirates suffered a complete defeat and have not been seen since.
This ark is a huge turning point for many diffrent characters mainly Luffy as his brother who he looked up you and adored was killed to save his life. Ace's death provided a much needed realization to luffy, which showed him thats he was not as strong as he thought