If you look up “anticlimax” in the dictionary, you'll see this story arc of That Time I Got Reincarnated as a Slime listed as an example. Honestly, the arc has so much going for it with Mariabell as a villain.

She was built up as a looming threat in the previous season and is one Rimuru is initially completely unaware of and unprepared for. Not only does she have the ability to mind control a seemingly limitless number of people—including powerful reincarnators like Yuuki—but she was also a puppet master back on Earth, a woman who ruled Europe from the shadows. When you compare her to a normal office worker like Rimuru, it feels like he might be out of his depth—especially when it comes to back-room politics and underworld crime.

The story is set up in such a way that it could easily be a high-tension thriller. Anyone could be under Mariabell's control. Some of Rimuru's greatest and most powerful allies could be forced to fight them—leaving the Slime severely handicapped by his reluctance to hurt his friends. Likewise, Mariabell's real-world experience could have her outmaneuvering even Raphael, leaving Rimuru perpetually on the back foot. In other words, this could easily have been a story arc with tension we haven't felt since the bloody attack on Tempest. Instead, we get 12 episodes of wasted potential.

The first four episodes largely focus on Rimuru playing around with the game balance in his dungeon—even messing around in it via an avatar body. It does little for the overall plot, aside from the scenes that further flesh out Mariabell's past and her current plans.

The next three episodes are a kind of political thriller in which Rimuru attempts to add Tempest to a pseudo-United Nations, while Mariabell schemes to discredit the monster nation and drum up anti-monster fervor among the human kingdoms. While this could have been interesting, Rimuru more or less stumbles into victory by sheer chance—not only through speeches and Raphael-aided treaty-writing but also by catching a teleporting bullet meant to frame them for a royal assassination. And to be clear, he has no idea that he is being set up until after everything has been resolved.

But where the arc is at its worst is in the back half of the cour where Mariabell attempts to confront Rimuru head-on after luring them into a trap. The big issue here is that Mariabell is a schemer, not a fighter. Her fight against Rimuru lasts seconds—and even the subsequent fight against Yuuki lasts only a few minutes longer. Never does Rimuru struggle—not once do Mariabell's traps meaningfully weaken Rimuru, nor does it ever feel like Yuuki has any chance at winning. Even the subsequent fight against the Chaos Dragon—the twisted resurrection of Milim's childhood friend—is a joke of a fight that Milim ends in a single blow. Sure, the fights are big, flashy, and colorful, but there is no tension behind the visuals—Rimuru's complete victory over any trials is treated as a foregone conclusion.

In the end, the moral of this season seems to be that Mariabell was a fool—someone who couldn't grasp how powerful Rimuru was (not to mention the limits of Yuuki's greed). Rather than a true rival, she was someone who dreamed too big and was crushed in the name of her ambition. As the danger was so easily overcome—especially after all the buildup—the entire arc feels like a waste of time.

And worse still, at the end of the arc, we are left with a villain vacuum. After his beatdown at Rimuru's hands, Yuuki hardly qualifies as a threat, and Granville, who was already playing second fiddle to Mariabell, seems even less of one in comparison. If there isn't any danger to our main character—be it physical or emotional—all that's left is the power-fantasy catharsis of watching our cute, mass-murdering Slime win effortlessly yet again.

All in all, this first cour of That Time I Got Reincarnated as a Slime Season 4 looks above average visually (especially in the big fight scenes) and has a fun opening and ending song to supplement its already decent soundtrack. But that can't change the fact that this arc serves as the series' greatest anticlimax to date. It honestly feels like the series is simply going through the motions at this point—setting up pins only for Rimuru to knock them down lackadaisically (and often accidentally). If you're a longtime fan who wants nothing more than to see Rimuru being cute and some flashy action scenes, you'll probably be fine with this half-season. But the fact that it could have been more—should have been more—makes me depressed more than anything else.