Eren and the select members of Survey Corps are in his father's basement. They read the first of the three books offering big revelations of their past.
Chapter 86 of "Attack on Titan" is already available in Manga Stream and it tells the story about the young Grisha and her sister Fay. The siblings wanted to take a closer look at the blimp but ended up in a restricted area. Her sister was taken away and Grisha was physically punished. A few days later, Fay was found in the river banks dead.
Grisha was outraged that there was no clear explanation on what happened. Worse, her father and mother were scolded because they allowed their children to roam outside the ghetto. Grisha wondered why his parents just took the explanation of Fay's death without any complaints.
His father told Grisha of their past as the heir of the Titan through their deal with the devil. With the use of their power, they destroyed the Marley Empire while establishing Eldia. The Eldian people continued their persecution of everyone until they were destroyed by the Marley uprising after controlling the most Titans.
Grisha continued his story as a doctor where he was eventually recruited as part of the Eldian rebels. Through the resistance Grisha met Dina, one of the royal family members. The got married a year later and their first born was named Zeke.
The Marley Empire announced their intention to go to war against the royal family living in Paradis Island with the use of Eldian children. The resistance decided to use Zeke for infiltration purposes but Zeke ultimately betrayed his parents. The couple was sent to Paradis as punishment.
The Manga Helpers forum discussion on the latest chapter focuses on who could be the first Titan. Although the story tells that Ymir Fritz made a deal with the devil, the actual devil could be a persona attributed to the creator of Titans.
The future chapters will offer more revelations as Eren and the rest of the Survey Corpse will open two more books. Grisha's basement will also reveal how they were able to infiltrate the walls and become part of Eren's civilization.