Chapter 1021 is titled "Demonio".
Black Maria overwhelms Robin with a combination of fire, spider web, and physical attacks and begins questioning Robin's usefulness again. However, Robin is able to use a Fish-Man Karate attack, which she learned from Koala and Hack on Baltigo, to put out the fire. Her giant clone then takes the form of a demon, and it seizes hold of Black Maria and smashes her into the ground, defeating her. Brook then finishes off Maria's subordinates with a single music attack.
On the Wano Country mainland, Luffy gorges on meat that a nearby Caribou had stored and regains his strength. Meanwhile, Momonosuke begs Shinobu to use her Juku Juku ability to age his body into an adult so he can become a bigger dragon, even though Shinobu warns that this technique is irreversible.
Unfazed by Robin's new attack, Black Maria simply says that that giant form of hers must take a lot of energy out of her. Robin tells her not to worry as she plans on finishing their fight quickly, sending her arms at her with the move Sea Serpent Snapdragon. Robin manages to grab Black Maria by her spider legs, but before she can grab her head, Black Maria strikes her giant arms, causing Robin's main body to feel the pain.
Recognizing this weakness, Black Maria jumps up onto the ceiling while swinging her Wanyudo to damage the arms just as Robin notices poison on Black Maria's spider legs, causing her to pull her arms back. This does allow Black Maria the opportunity to restrain Robin's giant form with Marianet, and then use Oshizumaria to leave her giant form open and defenseless. She equips her brass knuckles again saying she is a fighter at heart and proceeds to pummel Robin's giant form, causing her to feel the pain herself.
Robin manages to deactivate her giant form but tries to use the giant arms to grab Black Maria again, only this time Black Maria uses Ikidomaria to block them and uses her spider part, which can breathe fire, to set her highly flammable webs on fire, once again setting the floor on fire with her Atatamaria. Black Maria mocks Robin's claim that she would finish the fight quickly as she traps her in her webs and claims it is what she gets for defending Sanji, who she still calls a disgrace. Black Maria asks if Robin can sprout anymore "flowers" in this sea of flames and if she plans on asking Brook for help, though Brook simply tells Black Maria to stop saying stupid things and to make herself scarce.
Black Maria pays no heed and proceeds to pummel Robin's main body all the while she reiterates how she called her empty baggage with only her mind being of true worth. As Robin endures Black Maria's attacks, she thinks back to her time with the Revolutionary Army, and how Koala and Sabo wanted to teach her Fish-Man Karate and Ryusoken, respectively. Hack told them to back off as Robin's submission holds are plenty strong enough, but Robin decides to learn about using her palms, which delights the two.
Robin remembers how she used her palms when the crew was going to Fish-Man Island and the lessons they gave her. With it, she uses Mil Fleur: Fish-Man Karate: Giganteum, creating a huge hand that strikes the ceiling above. Black Maria at first thinks she missed since it came from behind her, but quickly realizes the attack was meant to put out the fire she started. Robin then displays a brand new form called Demonio Fleur, becoming a dark-colored demon with wings.
Black Maria immediately calls Robin's new form a devil, which Robin does not deny since it will help her protect those that count on her. Robin then grabs Black Maria's entire body and uses Grand Jacuzzi Clutch to knock her unconscious. Black Maria's subordinates, who hear her screams, peek out from the wall of ice Brook made and are horrified to see her defeated with the demonic looking Robin standing over her, scaring all of them as well. Brook takes the opportunity to finish them off with Phrase d'armes: Orchestra. He walks over to Robin and helps her up as a Mary is shown to have observed the fight.
Down at Tokage Port, Caribou, who has given Luffy a month's supply of food he kept stored in his body with his Devil Fruit powers, demands that he and the Heart Pirates better pay him back for it. The Heart Pirates ask Luffy if he can tell them about the status of their captain and Bepo's group, but he isn't able to properly tell them due to stuffing his face.
Off to the side, Momonosuke asks Shinobu a favor, which is revealed to be him asking her to use her Devil Fruit powers to mature his body into that of an adult and therefore, become a bigger dragon. Shinobu refuses out of concern of how dangerous it is on Onigashima, and on top of that, if she does use her powers on him, he won't revert back and he will still have the heart and mind of a child. Momonosuke states he doesn't care about that, saying that if he doesn't take action now, all of Wano will be destroyed.
- The fight between Robin and Black Maria concludes.
- Robin is shown to be able to extend and twist her arms similar to Luffy's usage of the Gomu Gomu no Mi.
- Black Maria's spider half is able to generate fire from its mouth.
- Robin uses a new technique she learned from Sabo and Koala.
- The technique is part of the Fish-Man Karate art.
- Robin creates a massive demonic representation of herself, and defeats Black Maria.
- Brook defeats Black Maria's subordinates.
- The Heart Pirates and Caribou have provided food for Luffy to recover.
- Momonosuke talks with Shinobu privately.
- He wants Shinobu to use her ability to mature his body into that of an adult so he can become a bigger dragon.