ONE PIECE GOLDThis is going to be a short theory based on what we saw on chapter 787,when Zoro uses Busoshouku Haki on his swords.
As you all can see,when he activates his Haki,it gets out of the sword a little.What does that have to do with Rayleigh?
Well,he is called Dark King so I guess that like Zoro’s Haki was showm out of his sword when it was covering his sword I think Rayleigh was able to create an aura of Busoshouku around him,covering the air,the ground and what’s beneath it,thus the name Dark King.
Thanks for reading
*Theory by Pedro5822