As soon as Albedo departs from E-Rantel for her special envoy mission, Ainz takes a trip of his own, using Gate to slip into the Imperial capital of Arwintar with his new pal Ainzach. He’s there to begin enacting his plan to recruit adventurers who actually go on adventures, and also checks in on his old pal Flugel, giving him The Book of the Dead from Earth and just a tiny taste of the secrets it holds.
Ainzach is actually the one to suggest Ainz himself fight in the coliseum in order to advertise his recruiting plan, but you also get the feeling Ainz has been itching for a good old-fashioned fight. He meets with the Warrior King’s tiny-eyed promoter Osk, who has not only thoroughly researched Ainz, but uses a Bunny Person Maid’s heightened senses to gauge his power.
As you’d expect, fighting one-on-one with any other individual would be too short a fight, so Ainz not only agrees not to use magic, but also commits to not using magical items (other than his own body and its built-in attributes). Also, if Albedo knew her beloved Ainz-sama would be fighting in an arena while dressed so cool, she’d probably teleport right over there.
Overlord is great at quickly building up grizzled, noble characters, and the Warrior King, a War Troll named Go Gin, is as grizzled and noble as they get. He and Ainz actually exchange some banter and laughter before getting down to it, when we see that the very best warrior an empire can offer is absolutely no match for even an Ainz Superleggera.
When Ainz indicates he’s about to go in for the kill, the Warrior King removes all of his armor. He knows Ainz has been holding back, and asks him if he’s really so weak. Ainz assures him that it’s more a matter of him being so strong. But he acquiesces to the King’s demand that he show him at least a “fraction” of his full strength.
Once he does, Ainz’s defense is so strong, a flurry of blows from the great Warrior King’s massive club bounces harmlessly off him, allowing him to get within point blank range and casually plunge his blade into the Warrior King’s chest, killing him. Ainz then flips on his magic mic and announces his adventurer recruitment plan.
To do so before a stone-silent crowd who just saw their beloved champion so comically bested shows Ainz’s lack of showmanship. It turns out he also didn’t come just to mess with El-Nix, though he is irked that the emperor rooted loudly for the Warrior King throughout the match.
Ainz’s intent hardly matters, however. By so easily killing the empire’s greatest warrior—then bringing him back to life during his speech—El-Nix is convinced right then and there that surrender is the only way to save his empire and its people. When Ainz visits his box after the bout, El-Nix offers his empire to Ainz as a vassal state.
Hilariously, Inner Ainz has no idea what that means, so makes up an excuse to skedaddle with haste. But El-Nix even interprets that as some kind of shrewed 5D-Chess move. At least one of his generals suggests that maybe Ainz wasn’t really thinking of anything…which was the truth!