I Left My A-Rank Party episode 21, titled Tenebre, aired on June 8, 2025, and depicted the actual planning involved before the actual expedition into the Achromatic Darkness dungeon.
Although episode 20 mentioned that the Clovers would need to travel all the way to Finis to enter and actually conquer the Achromatic Darkness dungeon, I Left My A-Rank Party episode 21 instead revealed something different. It showed that all of the dungeons around the world were connected via a subspace, serving as gateways between two worlds and different dungeons.
Disclaimer: This article contains heavy spoilers for I Left My A-Rank Party episode 21.
I Left My A-Rank Party episode 21 highlights

I Left My A-Rank Party episode 21 picked up where episode 20 left off with Yuke, Benwood, and Prince Mastoma. They were discussing their plan to combat the apocalyptic scenario arising from the Achromatic Darkness dungeon's broken seal.
Benwood goes on to suggest sending an investigation party before the Clovers' expedition, only for Yuke to reject the proposition, mentioning how it'll just put more lives at risk.
Eventually, Yuke decides to conquer the Achromatic Darkness and close it once and for all instead of actually sealing it. Although Benwood doubts the actual feasibility of the plan, he agrees to the plan.

That same night, Yuke receives a revelation in his dreams, realizing that the dungeons across the world serve as gateways between different worlds as well as each other. This makes it so that every dungeon is possibly connected to each other.
Yuke then barges into Prince Mastoma's residence to announce how he'll venture into Salmutaria's Royal Mausoleum dungeon alone and eventually conquer the Achromatic Darkness dungeon from the inside. Mastoma simply asks Yuke to actually consult with his "wives" before going on a perilous solo mission into one of the most treacherous dungeons there is.

Although Yuke is hesitant towards endangering the Clover Party members' lives, all of them hold their respective positions. They eventually agreed to enter the dungeons together as a cohesive party.
Unfortunately, the next day, it is revealed that the barrier surrounding the dungeons transforms individuals into shadow stalkers, putting the lives of the Clovers in jeopardy.
Fortunately, Yuke's long-lost uncle and former hero, Saga, appears and reveals that the brand Yuke received from Persephone is actually a ward allowing him to safely traverse the barrier encapsulating the dungeons. Apparently, the effect of the brand could be shared with Yuke's close associates, essentially allowing the Clovers to enter the dungeon like they previously intended.
Saga then decides to head back to Finis alongside Benwood to reinforce the town and prepare for any unforeseen calamities. The episode subsequently ended with Yuke and the rest of the Clover Party preparing to enter the Royal Mausoleum.
Final Thoughts
I Left My A-Rank Party episode 21 served as an intermission between the Salmutarian expedition as well as the upcoming final act of the series. The Achromatic Darkness dungeon, alongside Yuke's status as a hero, was being mentioned throughout the previous episodes. I Left My A-Rank Party episode 21, and the subsequent episodes will finally reveal the significance that they hold.
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