Silvers Rayleigh was once a member of Gol. D. Roger’s crew and he has been able to reach Raftel. He also trained Luffy during the two-years time skip and we all saw his incredible strength when he fought against Kizaru in the Sabaody islands.
In addition, Oda revealed that Rayleigh’s King’s Haki is as powerful as Shanks’ one. If you think about this is amazing, since Shanks is a Yonko, while Rayleigh is a “normal” pirate, but it makes sense because they were both member of Roger’s crew and Rayleigh is older than Shanks, so maybe Rayleigh taught Shanks how to use it.
Moreover, we also know that Rayleigh is named “The Dark King”. Why?
Well, in my opinion Rayleigh is able to use a special kind of Haki that I call “Advanced Haki”: this should be the strongest type of Haki ever shown and it is supposed to be used both for attacking and defending; for this reason, I think that Rayleigh uses this kind of Haki to create black forms and shapes.
I’m convinced of the fact that, when he fights at his full power, his Advanced Haki covers a big zone around the battlefield, so that his enemies can’t see the sun and, in that DARK place, Rayleigh is the KING because he created it and in there he can do what he wants; that’s my explanation to his nickname.
Furthermore, as far as his powers are concerned, in that black zone everyone is completely subdued to Rayleigh’s will: you can’t get in if you’re outside, you can’t get out if you’re inside, as long as he decides to make you stay in or out.
In addition, considering Rayleigh himself, he’s alone, he doesn’t have a crew or an army, so why the Marine didn’t try to catch him after Marineford events?
Well, when Garp was talking about Whitebeard and Rayleigh, he used the expression “two legends” to define them, so that we can imagine that Rayleigh’s powers are quite the same as Whitebeard’s ones; moreover, Garp also said “we’ll lose an unimaginable amount of men”, like he was conscious of Rayleigh’s terrible strength.
Another important event is when Kizaru meets Rayleigh in Sabaody islands: he doesn’t try to catch him, so that we can understand from this that an Admiral isn’t enough to catch rayleigh; Kizaru told him “we will get you when we’re ready”, but two years passed since that moment and they’re still not ready. (Or they just don’t want to be ready?)
In conclusion, since Rayleigh was Roger’s vice-captain, he should be AT LEAST near to his level (and so also to Whitebeard’s level), but maybe he’s also at their same level; this would mean that his power is something so strong to be compared to a Yonko, so even more than an Admiral!
In any case, Rayleigh and Shanks have in common the use of something never seen before: the “Advanced Haki”.