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Kaido and Big Mom unleash a massive combined attack called "Hakai" (破壊 ). This attack is clearly a parallel to the one used by Dorry and Brogy at the end of the Little Garden arc, which can be seen from their poses and sound effects. It is a powerful enough attack to open a hole in a massive Sea King.
The parallel becomes even more evident if we remember that Dorry and Brogy's attack was called "Hakoku," with the name "Hakai" being used here making a direct reference to the use of the kanji. We saw Big Mom use her own variant of "Hakoku" on the Whole Cake Island, she called it "Ikoku," so she is used to using these types of attacks from Elbaf. "Hakoku" can be translated as "Country of Warriors" (alternatively also as lords, sovereigns, etc.), while "Ikoku" can be translated to "Country of the Powerful." However, in the case of "Hakai," the "koku" part (which indicates country) is replaced by the sea kanji, turning the word into "Sea of Warriors." The reference here is that the "warriors" part is referring to King's Haki, at least in this scene, since it uses the same kanji (Hakai - Sea of Warriors. Haoshoku - Haki of the King. As well as in the word "Haki" itself, which can be translated literally as "conqueror's ki," or more simply: ambition, determination), with the sea part in "Hakai" referring to the influence of the Four Emperors on the world's seas.
But the reference does not stop here! There is an even deeper layer in the wordplay here, as expected from Oda. It is possible to read the kanji of "Hakoku" in the same way as "Nation of Warriors," but it has its own meaning of "domination." Similarly, by reading "Hakai," it is possible to notice the kanji "Nation of Warriors," but it can also refer to the absolute force of "destruction." BUT WAIT, THERE'S MORE! It's an even smarter wordplay with the words, this combined attack, in and of itself, is the combination of Big Mom and Kaido's names, with the "Ha" being half of "Haha," the Japanese translation of "Mom," and the "Kai" being half of "Kaido," making this attack a combination of their names.
With that, we learn that the attack is solely based on HAOSHOKU... The question is, depending on the way Kaidou and Big Mom are, can the attack be exponentially increased in power?
We know that Kaido's Haoshoku standard is at the "heaven" division level and on par with Roger and Whitebeard's (as Yamato stated).

Yamato comparing both feats:

Impressed with this level of Acoc? So don't be surprised because Kaidou in his Shurron Hakkeu mode surpassed this level.
Kaido Conqueror's haki grows stronger with every minute he spends in Shurron Hakke.

With that we can define that Kaidou's Haki used in chapter 1026 that shared the skies with Luffy < Haki in Shurron Hakke mode that was even stronger.
As I already explained that Hakai is based on pure Conqueror Haki, and Kaidou manages to further increase his Haoshoku level, we can say that Hakai would be even stronger if Kaidou combined with Big Mom while he was in his "drunken mode"...

While Big Mom can somehow increase her Haki as well, it could be through consuming years of her life or something else, but the fact remains that Kaido's Haki can be fortified.
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