Zeke Yaeger has finally revealed his hand in Attack on Titan season 4 is hurtling towards its endgame, and with a hefty chunk of manga still to adapt, the anime's future remains unclear at present. The situation on Paradis Island is just as unpredictable, as Eren Yaeger has effectively taken control of the island alongside like-minded ers who believe war against Marley is the only plausible course of action. Before Eren's coup, Paradis was cautiously working with his brother Zeke, otherwise known as the Beast Titan, and the elder sibling is now on the island under Levi's strict guard.
The most recent installment of Attack on Titan season 4 begins with Eren attempting to uncover Zeke's location, but the episode was curtailed by real life events, as an earthquake struck Japan and the broadcast was halted. Attack on Titan will resume March 21st airing the interrupted episode back-to-back with episode 15, which was originally scheduled for that slot. When the episode does finally air, Attack on Titan will adapt Zeke's grand plan from Hajime Isayama's original manga series.
To understand Zeke Yaeger's plan in Attack on Titan, it's important to consider the limited resources on Paradis Island. The Survey Corps didn't even know what coffee was before Marley came along. After Yelena brokered peace with Paradis on Zeke's behalf, she brought along a treasure trove of new foods and drinks from the outside world for Eldians to discover, one of which was an especially delicious wine reserved only for higher-ups. Before shipping, however, these bottles had been laced with the spinal fluid of Zeke Yaeger. On Zeke's behalf, Yelena then ensured her most loyal followers were given jobs in catering, restaurants, etc. so they could distribute the spiked wine without suspicion. Those whom Yelena believed had grown too close to the Paradis residents (such as Niccolo and Onyankopon) were not told about the true nature of the wine.
The wine's transformation properties derive from Titan powers specific to Zeke. Whether it's the Beast Titan's powers.
To gain the islanders' trust, Zeke itted that a gaseous variant of his spinal fluid was used to turn the entire village of Ragako into Titans. The revelation lowered the military's guard enough for Zeke to slip a seemingly innocuous lie into his tale - that after inhaling the Beast Titan's spinal fluid, the villagers were "paralyzed." This is complete fabrication, but fools the likes of Commander Pyxis and Zachary into believing there'll be an early warning if Zeke ever tries to poison them. Had they known the fluid is unnoticeable, they might not have guzzled down Marley's wine quite so readily.
Zeke's intentions remain a mystery in Attack on Titan season 4, but plenty of clues have been left over recent episodes. Since Zeke betrayed Marley, he's definitely not on their side. The current Beast Titan also has only a year to live, which suggests Zeke isn't motivated by self-interest. More likely, Zeke's true plan to save Eldia is different from the Rumbling idea he planned with Paradis Island. Zeke knew the Paradis military would never risk allowing him and Eren Yaeger to make (just in case the Beast's intentions weren't genuine), so he needed to ensure a meeting with Eren could happen and the full power of the Founding Titan could be unleashed, enacting the plan they've cooked up together in secret.