Eren Jaeger and his half-brother Zeke are either going to change or destroy the world when Attack on Titan returns for the second half of its final season. If Eren, who holds the Founding Titan, and Zeke, who possesses the royal blood necessary to use it, manage to come into contact, they will gain omnipotent control over all Titans and Subjects of Ymir, aka the Eldian people. As they prepare to put their plans into action, as described by Yelena, the two brothers are about to change the world. The relationship between Eren and Zeke is going to be vital to the final season and the fate of every character in the series.
Half-brothers Eren and Zeke share the same father, Grisha Jaeger. While Zeke was raised in the Liberio Internment Zone by his radical father and groomed to be the savior of the Eldian people, Eren was raised in peace on Paradis Island. Unaware of the hatred for Eldians that Zeke grew up with, Eren was a fairly normal child until Marley's first attack on Paradis Island at the beginning of the series forced Grisha to turn Eren into a Titan and direct him toward proof of the truth of the world. While the brothers took very different paths, they each became obsessed with their father's goal of freeing the Eldian people from imprisonment and bigotry.
The Jaeger brothers did not meet until the end of Season 3, with Eren unaware that the enemy Titan before him was his older half-brother. While Zeke has always wanted to save Eren from the brainwashing of their father, Eren has only recently begun working with Zeke, the two having met in secret prior to Eren's attack on Marley in the final season. In fact, Eren's plan to launch a preemptive strike on Marley was not only to buy time for Paradis Island but also as a way to extract Zeke from enemy territory.
Eren appears to have come around on Zeke's Euthanasia Plan, which entails the sterilization of the Eldian people so that, unable to reproduce, they will peacefully die out over the next century or so. As if wiping out their own race weren't bad enough, Eren also plans to use the power of the Founding Titan to unleash the millions of Colossus Titans inside the Walls on Paradis Island as a display of strength, preventing other countries from attacking as the Eldians peacefully die out. This horrific, genocidal plan has been the focus of the Final Season as various groups fight to either prevent it or make it happen.
Surprisingly, Eren has agreed to take Zeke's lead on this plan, persuaded by Zeke's argument that the greatest kindness that could ever be given to Eldians would be to never have been born in the first place. While it's a tad suspicious that Eren was so easily swayed by Zeke, the two must work together in order to wield the power of the Founding Titan, so their cooperation is also a thing born of necessity. Each of the Jaeger brothers wants to save the Eldian people, and if working together is the only way to do it, then being in agreement or even accepting a compromise would be a logical step.
When the Final Season: Part Two picks up where it left off, Eren will find himself facing down the Marleyan army alone while Zeke has been carried to parts unknown by a Titan after being blown to bits by a Thunder Spear. While Shiganshina appears to be their previously agreed-upon meetup spot, it is also now a warzone, meaning their mission of coming into contact will certainly be a drawn-out, bloody battle filled with classic AOT chaos and carnage. The Jaeger brothers will have to rely on the strength of their will and their faith in each other if they intend to carry out their plan with a full-blown war raging around them.
It's not clear exactly what will happen if Eren and Zeke do manage to come into contact, other than that they will control the Founding Titan. As they are not bound by King Fritz's vow renouncing war, they will be free to wield the Founding Titan in any way they see fit, which would be game over for anyone who had hoped to stop them. It's been well-established that unleashing the Wall Titans will drown the world in blood, and the Jaeger brothers have already shown that they will do whatever it takes, even betraying and killing, in order to carry out their plan. They are the only ones who understand each other, and together, they just might destroy the world.