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So as the name and the picture suggests this is a theory on Rayleigh’s powers…
As we all know Rayleigh’s alias is the Dark King. My theory is that he got this name by his top of world(once upon a time, now weakend due to old age) CoA:hardening(maybe stronger than or epual to the pk).
You can see the toughness of his haki in instances like in the fight with Kizaru he coated his sword in CoA so that it wont get cut by Kizaru’s sword(since it is concentrated beam of lighting so it works like laser)

and also the speed at which he came to kick Kizaru from attacking Zoro was also achieved by using CoA on his feet and then kicking the ground (like instant movement in uq holder or negima if you know about those two)

in the gif above it shows the dust rising from his feet…
also when he came to Marineford shot bullets with bare hands and then shattering the cannonballs although this can be acheived by the kuja also,the speciality of Rayleigh lies in shooting multiple bullets with accuracy and CoA embbedded into them

so instead of a baseless statement that “the demon” resides in Rayleigh I suppose he was called the Dark King because he had insane CoA: hardening levels like being able to put three layers on top of his body or atleast he was the first person who introduced the common world to “hardening” or CoA since haki was developed by the kujas. As we have seen in the series the marines don’t tend to change peoples titles such as Zoro is still called pirate hunter even though he is a pirate and he never actually intended to be a pirate hunter.So Rayleigh was also given the name Dark king by some dumbass marine or commoner cause he was black(no racism intended)

*Theory by gamergautham

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