The Hassaikai is a giant, heavy kanabo befitting Kaido's size. It is made of a solid black metal, with a round cross-section, and features sharp spikes along its upper, bulkier half. The latter tapers somewhat toward the slenderer, spikeless handle, which itself ends in a suspension ring that serves as a pommel.
Kaidou utilizes this club for the vast majority of his melee attacks. Brandishing the weapon with his massive strength while channeling his Haoshoku Haki through it, Kaidou can severely injure and send foes flying far distances with immense force, in just single blows. He wields the weapon both one- and two-handed, the latter especially for his more powerful attacks while in his half-dragon form. Defensively, Kaidou can also use his weapon as a means to deflect powerful attacks.
Using single swings of the club, he respectively dealt finishing blows to the mighty Kozuki Oden two decades ago (the man left severely injured, bleeding, and unconscious from the hit) as well as to a Gear Fourth-using Monkey D. Luffy in the present, breaking past the tremendous Haki defense of the latter's empowered form and, again, rendering the foe unconscious.
Raimei Hakke (雷鳴八卦,Raimei Hakke?, literally meaning "Thunderclap Eight Trigrams"): Kaidou charges toward his opponent at blinding speed, swinging his Hassaikai horizontally, one-handed to strike them with devastating force. The attack is so fast that, even after seeing the future, Luffy was only half-successful in dodging it. Additionally, Kaidou can imbue his kanabo with both Haoshoku and Busoshoku Haki for the attack, which causes a visible trail of dark lightning to exude from the weapon. When first used against Luffy in his Boundman state, Raimei Hakke knocked the 1.5-billion-bounty pirate unconscious in one hit. An imitation of the Raimei Hakke is used by Kaidou's child, Yamato, to devastating effect, albeit purportedly weaker than Kaidou's; when Yamato in her half-Makami form did have a clash with Kaidou in his half-dragon form—using their respective versions of Raimei Hakke—their attacks seemed evenly matched, but Yamato ultimately ended up taking more damage despite protecting herself. The "Bāguà" (八卦,Hakke?, "Eight Trigrams" in English) are eight tripartite symbols used in Taoist cosmology to represent the fundamentals of reality, based on eight interrelated concepts. The Viz manga calls the attack Thunder Bagua.
Hakai (覇海,Hakai?, literally meaning "Supremacy Sea"): A combined attack of Kaidou and Big Mom. Standing side to side, Kaidou in his Human-Beast form swings forward his Hassaikai while Big Mom simultaneously swings Napoleon in its Cognac form. The combined power of the Emperor's strikes creates a powerful, explosion-like wave of force that quickly travels forward and absolutely devastates everything in its path; the attack was shown to dwarf the island of Onigashima, and Roronoa Zoro was heavily injured even by just attempting to block the attack for a moment. This move is very similar to Dorry and Brogy's combined Hakoku technique, featuring even the same poses. In the Viz manga, this is called Conquest of the Sea.
Kosanze Ragnaraku (降三世引奈落,Kōsanze Ragunaraku?, literally meaning "Conquered Three Worlds, Attracting Hell"): An attack performed by Kaidou in his Human-Beast Form. Wielding his Hassaikai two-handed, he jumps into the air and quickly spins his weapon above his head as he imbues it with Haoshoku Haki, making it exude black lightning. He then, while descending, swings his club down with great force, smashing his foe into the ground. The move's first demonstration came against Luffy, who took the blow straight on and was knocked out by it for a brief period. The first part of the attack's name comes from Trailokyavijaya (Gōzanze Myōō, "Conqueror of Three Worlds"), one of the Five Wisdom Kings in Vajrayāna Buddhism. The name's second part refers to the Buddhist "hell", called Naraku (奈落,Naraku?) in Japanese; Ragu (引,Ragu?) is prepended to make the reading play on Ragunaroku (ラグナロク,Ragunaroku?), the Japanese rendering of Ragnarök (the apocalypse in Norse mythology). In the Viz manga, this is called Conquerer of Three Worlds Ragnaraku.
Kongo Kabura (金剛鏑,Kongō Kabura?, literally meaning "Adamantine Turnip"): An attack used by Kaidou in his Human-Beast Form, where he swings forward Hassaikai to launch a long-ranged, projectile-like shockwave at an opponent. When first used by Kaidou in his fight against Yamato, the attack proved powerful enough to blow away the latter in her Human-Beast Form, despite Yamato blocking with her own kanabo.Kongo Kabura appears to be a stronger version of Yamato's Narikabura technique. The word kongō (金剛,kongō?) can variously refer to adamantine, diamond, or the vajra from Indian mythology, while kabura (鏑,kabura?) comes from "kabura-ya", a type of traditional Japanese arrow with a turnip-shaped whistle attached. In the Viz manga, this attack is called Vajra Arrow.
Kaidou already seemed to wield this very kanabo when he joined the Rocks Pirates over 40 years ago as an apprentice.
This weapon is named after the "eight precepts" from Buddhism, a set of eight moral guidelines practiced by lay devotees on observance days and festivals, usually to aid in the process of meditation. They are related to the Buddhist concept of the "five precepts".
According to Oda, Hassaikai does not have a rank (位列,iretsu?), though if Kaidou were to leave the weapon behind it would be called a "legend".
Like the weapons of other characters, Oda has drawn a joke rendering of Kaidou's Hassaikai anthropomorphized, taking the form of a bulky, top-heavy old man with long hair, insistent on "not going outside".