He has mastered Haoshoku Haki (Conqueror’s Haki) to the extent that he can render a specific target completely unconscious. He is able to do this without adversely affecting anybody else near the initial target. He disabled an entire hall room of hostiles with this power, save for the Kid Pirates and Heart Pirates.
Only very strong and determined individuals can resist from being knocked unconscious. One of Trafalgar Law’s crewmen was very close to being knocked out, and even Law and Eustass Kid were sweating slightly after enduring the blast.
Busoshoku Haki (Armament Haki) has an extremely high level of skill in Busoshoku Haki, allowing him to effectively bypass the powers of a Devil Fruit user and touching the “substantial body” beneath whatever protection the fruit provides as seen when he used Busoshoku Haki to kick and redirect an attack from Admiral Kizaru.
Rayleigh can utilize an advanced application of Busoshoku which enables him to emit his Haki a short distance without a medium.
He is also able to use a high-grade version of this technique, where Rayleigh can channel his Haki into his opponent, letting him destroy them from the inside out, which he first used to destroy Camie’s collar.
Kenbunshoku Haki (Observation Haki) can allow Rayleigh to sense the emotions and nature of others. The strength and mastery Rayleigh has over this kind of Haki is also shown by the fact that he can sense people all over the island he is currently on while most people can sense only in their immediate vicinity. Rayleigh could also use it to sense the strength of others on a wide scale. This was shown when Rayleigh utilized this Haki to sense that there were five hundred creatures on Rusukaina who were stronger than Luffy at that time.
The “Luffy Gear 4 Vs Rayleigh” filler scene in Episode 870 reminded us that Rayleigh is still one of the strongest beings in the entire One Piece World. Luffy Gear 4 Bounceman was just a punching bag to Rayleigh.
He probably could have one shotted Luffy just like Kaido did if he was a bit more serious. This could mean that Rayleigh is close to Yonko level if not Yonko level!
