Titans came from a single little slave girl,Ymir (God) that fell into a hollow tree and somehow became a titan. She became Queen and had a bunch of kids. After the she died, her kids were forced to eat her and they became titans. Only her descendants can become titans, and only if they eat the spinal fluid of a titan. All her decendants are also linked with a magical connection that transcends time and space (Paths); which is where Ymir lives and she literally hands crafts every titan body.
Eren's dad (Grisha) stole the epicenter of the Paths (The Attack Titan) because present Eren told him to after Eren had his head shot off in the current arc. Eren got sent to where Ymir lives after his half brother, Zeke (Beast Titan) caught his head and activated a power within the Attack Titan that only he as a member of the Royal Line can activate in the seconds Eren's decapitated head was still alive. So while Eren is in the Paths, he tells Grisha to murder Historia's family (The Attack Titan also shares past and future memories of anyone that has and will wield it, somehow he was able to activate future memories and use them to directly communicate) and tells Ymir to have all the titans to kill anyone not on their Island. After that Ymir heals Eren's body and he declares war.
The reason Eren's doing this is because of the attack on his village when he was a kid. He's butthurt since Reiner's country (Marley) is constantly persecuting his race (Eldians, Ymir's decendants) and is actively looking for a way to exterminate everyone on the Island (Paradis). He went to Marley for recon and found that the entire world wants him and his people exterminated completely since they can be titans. Marley breeds Eldians so they can be used as titans in warfare, but that's all they're useful for. Overall, they're hated because the Eldians dominated the world until one day their leaders decided to live in an isolationist society and settled on Paradis (They wiped all the settlers' memories, as well. It's another Royals-only titan power which only works on Eldians, though.). Eren and the Survey Corps actually attacked Reiner's village. Eren went rogue and asked for extraction after he attacked a Marlian leader that declared war on Paradis. Potatoe girl got killed.
TL
R The reason Eren is speshul is because he's the pope and now he and God are starting Armageddon.
