WARNING: The following contains spoilers for Season 4, Episode 7 of Attack on Titan, "Assault," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
With the power of the Attack and Founding Titans at his fingertips, Eren Jaeger was already the most powerful man on the planet before Season 4 of Attack on Titan even began. Now, seven episodes in, the man Willy Tybur singled out to Marley -- and the world -- as public enemy number one is deadlier than ever. As of Episode 7, "Assault," Eren adds the War Hammer Titan to his arsenal.
Not only does this make him a triple threat to all who oppose Paradis Island, but it also confirms an important piece of Titan lore, much of which is still riddled with questions and mysteries that the series may never answer in full.
Acquiring the power of a Titan Shifter, of which there are only nine, requires cannibalization of its current holder. Eren got both the Attack and Founding Titan from his father Grisha, as a child. Taking Eren into the woods, Grisha injected his son with Titan serum, turning him into a Pure Titan that then consumed Grisha to gain both powers at once. While he accidentally manifested the Attack Titan in Season 1, Eren didn't utilize the power of the Founding one until the end of Season 2, where he defended himself and Mikasa by unconsciously harnessing a fraction of the Founding Titan's dominion over other Titans, ready to eat them.
In Episode 7 of Season 4, Eren gains his third Shifter power purposefully for the first time. Using the strength of the Jaw Titan's mouth, he crunches through Lara Tybur's crystal defense system to drain her body of blood. The transference of the War Hammer Titan from her to Eren is visually indicated by a glowing swell in Eren's pupils.
What this makes absolutely clear is that there is no hard limit to how many Shifters one person can possess. Theoretically, it seems perfectly possible that Eren could, if he so wanted, acquire all Nine Titans for himself. That may well be his goal, too: he goes straight for Porco's Jaw Titan right after eating Lara, only to be halted by Reiner's Armored Titan.
But there's a problem. Willy's speech revealed that the Founding Titan has been bound by the will of the royal Fritz family for a century in order to make Paradis Island a true, peaceful paradise free from human or Titan tyranny. With the Founding Titan out of that lineage's hands, Marley fears nothing will stop Eren from unleashing it. However, as Rod Reiss demonstrated when he stole a small amount of the Titan's power, the all-powerful being is downright impossible for someone outside of the Fritz line to control, due to the family having a direct link back to the Founder, Ymir. Rod was only able to partially do so through contact with Historia Reiss, a Fritz descendant, while Eren through contact with Dina Fritz in her Abnormal Titan form.
Other than threatening Marley with the might of the Founding Titan being a pretty clear vote for absolute villainy, Eren does not have the capacity yet to fully utilize it, which is why, coupled with his years learning to master it, the Attack Titan is still his preferred means of combat. But, what'll be interesting for fans of the Attack on Titan anime to see is whether or not he'll make the War Hammer Titan part of his active arsenal, as no such restrictions have been shown to apply to it. Even if he doesn't, the War Hammer may instead act as a buff to his Attack Titan form, as drinking Rod's "Armor" serum did to it.
With Eren still in the midst of battle at the conclusion of Episode 7, there's also still a chance he could snatch up yet a fourth and even fifth Shifter power. It's all up to Reiner to put an end to his power hoarding.