Ainz's surprise appearance in the Colosseum where Jircniv was holding his secret "Anti-Ainz Alliance" meeting with members from the Slane Theocracy and his other co-conspirators is finally explained with a flashback.
While Overlord IV episode 4 seems to follow in the footsteps of the previous three episodes given the length of said flashback, and the detail to which we are privy to the conversation between Ainz and Pluton Ainzach, the Adventurer Guild Master. This conversation brings Ainz's awareness to a particular resident of Baharuth – the Warrior King Go Gin, whom Ainz wants to recruit.
Look Back to Look Ahead
While not getting off to the most exciting start, the episode quickly reminds viewers about what they came to see: Ainz Ooal Gown in full-blown combat. This fight is well-animated and also has the added importance of lore. The conversation with the human Ainzach reiterates one of the core themes of the current season, which is Ainz's deteriorating sense of humanity, and his sense of aloneness in the world of Yggrassil since his mysterious integration into the post-shutdown "meta". Ainz teleports himself, Ainzach and their transportation to the Imperial Capital Arwintar, which makes Ainzach incredibly uncomfortable at the premise of a king entering another country's borders secretly.
He quickly holds his tongue but Ainz tells him that he actually prefers it if he voices his opinions freely and encourages Ainzach to be more vocal. This carriage trip through the streets of Baharuth serves as exposition for various elements that have been set up in the series thus far.
This includes Ainz's decision to appear at the Colosseum, as well as his vision for the future of adventurers. It is during this trip that Ainz learns from Ainzach about the existence of Go Gin, the current holder of the title of Warrior King. Ainzach is connected to Go Gin's promoter, and schedules a meeting with them for the next day. The next morning, Ainz meets with Fluder, who tells him that Jircniv is aware of his betrayal. Ainz then asks Fluder for some information on the Warrior King, planning a strategy for his upcoming fight.
Discussing a Moment in the Ring
Despite being in disguise, Osk is well aware that he is in the presence of Ainz Ooal Gown. Osk is a merchant and promoter of the Warrior King. Ainz schedules his audience with the Warrior King in the form of a fight against him, explaining that his desire comes from a mere curiosity, and also adding the idea that his magic has its limits.
Osk asks about Ainz's intentions, before explaining that the Colosseum only permits melee combat. He also asks very warily about Ainz's genocidal magic, which Ainz explains as something which takes a long time to gather enough energy to conjure up. Osk, like Ainzach, is also very confused by the premise of a king fighting in the Colosseum, but he allows it. Once Ainz pledges to abide by the rules and engage solely with physical weapons, the fight is set, but Osk is extremely wary of his Majesty, Ainz's fearsome power.
Watch Your Humanity
There are certain things that Ainz himself doesn't realize make him particularly confusing as an entity in the overworld. These are elements that are reminders of his inherent humanity; or perhaps even reminders of the substance that links humans to the inhuman as living beings. His secret embarrassment at knowing nothing about politics, his decisions and his apparent incompetence as a political ruler are things he doesn't realize make him more human than not.
His decisions are incomprehensible to human and undead alike due to his special position of having been both, and actually being both at the same time. Sneaking into Baharuth, given the Sorcerer Kingdom's infamy would be international news, which Ainz seeks to avoid entirely. It isn't the best plan, especially given that he does engage in combat with the Warrior King in front of several fans and Jircniv El-Nix, the Emperor of Baharuth himself. However, Ainz simply lacks the understanding of manners in politics, and his desire to help the adventurers is his main priority at this point in time, so even if it's not the most put together plan, it can work.
It's a Conspiracy, It Must Be!
After the flashback, the anime takes viewers back to the Colosseum, where Jircniv is scrambling to put together an explanation for his little party. Once Ainz's battle commences, Jircniv shows his lack of loyalty by rooting for the Warrior King; doing so out of fear of the consequences of Ainz's knowing of his impending betrayal.
Unfortunately, Jircniv is actually smarter than Satoru Suzuki, the human being who is now Ainz Ooal Gown, so his understanding of the situation is such that places the incomprehensible motivations and actions of the undead far beyond the possible understanding of human beings. As such, Jircniv thinks that he has been outsmarted by a cunning demon, when in reality, Ainz wasn't even thinking too much about Jircniv's company, nor was his appearance in Baharuth some kind of test of loyalty. Ainz simply needed a way to talk to the Warrior King without attracting too much attention.
Look Momonga, No Hands!
One of the best things about this episode is the fight. The animation is sleek and something to behold. The fact that Ainz is definitely going to win the bout does not put a dampener on the affair; however, the fight does become much more interesting as a concept when you realize that Ainz placed a major handicap upon himself prior to the bout. This handicap was discussed during the meeting with Go Gin's promoter Osk, who explained the rules of the Colosseum, which also served as a reassurance to Osk and to the spectators present. It is common knowledge that Ainz Ooal Gown is capable of mass genocide at the invocation of a single spell, after all.
Ainz shows off high-speed attacks and proficiency with various weapons, overwhelming the Warrior King with ease. Realizing his imminent defeat, the Warrior King tells Ainz that he does not wish to fall to the overlord at a mere fraction of his power, but wants to experience the full brunt of Ainz's might. Understanding the war troll's wishes, Ainz begins casting as he lunges into a final onslaught. The fight concludes with Ainz stabbing the Warrior King square in his heart. Once the orc falls, Ainz proclaims his intention to recruit, train and nurture adventurers from all over the world, He then shows his mercy by reviving Go Gin, and as per the pre-match agreement, Go Gin is now one of the citizens of Nazarick and an officer chosen specifically by the ruler Ainz Ooal Gown himself.