WARNING: The following contains spoilers for Season 4, Episode 9 of Attack on Titan, now streaming on Crunchyroll, FUNimation, Hulu and Amazon Prime.
The political scope of Attack on Titan has expanded greatly. What was once an oversized zombie survival horror narrative became an expansive tale of international war, and come Season 4, the mighty Marley Empire has arisen as the primary antagonist to the people of Paradis Island. Eren and his friends are ready to strike back with the Attack and Founding Titans' power, but in Episode 9, they find another weapon dumped into their lap: a fresh set of Titan serums.
The Titan serum has appeared in the story a few times thus far. Once an Eldian is injected with it, they will irreversibly become a Pure Titan, with the potential to become one of the Nine Titans if circumstances allow. What will Levi Ackerman, Eren and the rest do with this extra ammo?
The History of the Titan Serum's Uses
The Titan serum has played a significant role in over the course of Attack on Titan. For one thing, flashbacks from Grisha Yeager's perspective made it clear that the serum was what transformed Eldians into Titans in the first place, and Marleyan agents turned these Titans loose on Paradis Island. Each and every Titan is really an Eldian man, woman or child, turned into a dog of war thanks to a simple needle to the spine. This serum creates "Pure" Titans that lose all human thought, desperately eating other people in an effort to try and obtain the power of the Nine and thus restore their humanity somewhat. Ymir was one such Titan until she ate Marcel Galliard by chance and became the new Jaw Titan wielder.
This Titan serum also allowed Rod Reiss (father of Historia Reiss) to transform into a Titan, intending to eat Eren and gain the power of the Founding Titan. Rod was foiled, but this sequence of events proved just how devastating one innocent-looking serum can be. Later still, Levi got his hands on a Titan serum and had to choose between giving it to Armin Arlert or Erwin Smith. Whoever got the injection would be saved from death, and that person could eat Bertholt and gain his Colossal Titan power. In the end, Levi chose Armin, who is now the Colossal Titan wielder, and Erwin died because of this choice.
What Does the Serum Mean for the Future?
Very few batches of the Titan serum were available on Paradis Island, but now, a whole collection has arrived thanks to Yelena, Onyankopon and the rest of the Anti-Marleyan Volunteers, all of whom are dissenters from the Marley Empire. Many of them lost their homelands and families to Marley's conquering armies, so it's possible that they're hoping the serum can turn their new Eldian allies into Titans that can crush Marley underfoot. As non-Eldians, these Marley dissenters can't become Titans themselves, which could be why they're acting as suppliers instead. They're giving Levi and Hange all sorts of intel, technological secrets and Titan serums to rapidly arm Paradis Island for modern warfare.
Will this serum be used at all, though? Who would volunteer to become a Pure Titan to wage a ground war on Marley? It would be cruel to force this serum on anyone -- doing so would make the Island commanders no better than the Marley officers who routinely turned Eldians into Pure Titans. However, if the Islanders become desperate and the war turns against them, they might try "drafting" Pure Titans like that anyway, which might divide the Eldians to the point of losing the war. Alternately, perhaps some of the more downtrodden Eldian civilians in Marley will volunteer to become Titans to release their pent-up frustration and helplessness in style. Yet another possibility is that Paradis Island's officers will confiscate the serums and keep them sealed away, unwilling to take such a risk.
And what if the serum makes things much worse, and the Titans go out of control? Perhaps the Island commanders would rather win the war via manpower alone, especially when modern Marleyan technology (such as zeppelins and trains) is available. Whatever the Eldians choose to do with it, one thing is certain: this serum is not to be taken lightly.