Kaido Harald's kid? Kaido said let's go beat him.
Well, obvious, Kaido DIDN'T KNOW he was Harald's Kid.
Kaido was born to the Queen of the Oni, which is why Yamato is the Oni-Hime, One Princess.
I think Harald led Cipher Pol to the Oni Tribe accidentally, while looking for other Giant Tribes/peoples with Giant Lineage, and then Cipher Pol massacred the Oni the same way they destroyed the Buccaneers.
The Buccaneers seemed to be a very peaceful people: if the WG found a Tribe of Kaidos, they'd slaughter them, no questions asked.