To reach the very top, which is a place currently held by "Hawk-eyes" Dracule Mihawk, Zoro has had to train his mind and body rigorously. With time, he's gotten much stronger and his way of the sword has improved as well. Against Mr. 1, Zoro learned to cut steel, against Ohm, he learned how to use projectile slashes, and against Kaku, Zoro tapped into the power of Ashura. He spent two years training under Mihawk and came back much stronger than ever before. However, his strength was still not enough to reach the top. In One Piece's Wano Country arc, however, Zoro has received his biggest power-up yet called the King of Hell Three Sword Style, or En-ou Santoryu, and it has made him much stronger than ever before.
Zoro's King of Hell Three Sword Style ties back to one of the biggest sword upgrades that he has ever received, the black blade Shusui. Zoro received this sword during the Thriller Bark arc after defeating Ryuma and carried it all the way to Wano, from where it was robbed in the first place. Hiyori, the princess of the country asked him to return the sword to the Land of Wano and in exchange, he received one of Kozuki Oden's strongest swords, Enma. This sword was described by Hitetsu as a blade that pierces even hell itself. As one of the 21 O Wazamono swords, Enma is a fantastic blade that anyone would love to wield but no one does. That may be strange but there's a massive reason behind it. Enma, although a great sword, has the ability to suck Haki from its wielder, making it incredibly difficult to wield. Average people would die just minutes after wielding it, however, Zoro managed to control Enma a lot better.
On the rooftop of Onigashima, he managed to impress even Kaido with Enma's power. Not only did Enma allow him to slice Kaido but it shocked the Yonko when he realized Zoro was channeling Conqueror's Haki through the blade. As Zoro was defeated by Kaido and Oda established the set-up for a one-on-one fight between Luffy and Kaido, Zoro then turned to King, one of Kaido's three right-hand men. Unsurprisingly, Zoro struggled in the fight against King and wasn't able to take him down by regulating his Haki. Realizing that he was getting nowhere by trying to control the blade, Zoro let Enma suck out as much Haki as it could freely, making him much stronger but only for a short period of time.
According to Zoro, this was enough to kill him in a matter of minutes, however, this is exactly what allowed him to overcome King the Wildfire. By not regulating his Haki anymore, Enma enabled Zoro to use high-level Ryuo, but that certainly isn't the most impressive part of this power-up. What's more impressive is that it also drew out how Conqueror's Haki and allowed him to coat his sword in it, hence awakening the King of Hell Three Sword Style.
"King of Hell" or "En-ou" comes directly from the blade's reputation to be the King of Hell. Its addition in the naming implies that Zoro's Three Sword Style is now made much stronger by the usage of Enma, as it draws out the maximum amount of Haki from his body. The fact that Zoro can now use advanced Conqueror's Haki makes him one of the strongest characters in the entire story and takes him one step closer to achieving his dream of surpassing Dracule Mihawk. In his fight against King, Zoro was seen making use of his King of Hell Three Sword Style and fans got a clear idea of how strong he truly is.
Using the King of Hell Three Sword Style, Zoro was able to cut King when his defenses were very low and chink away his armor gradually. King, while strong, was simply no match for the monstrous power of Zoro's new sword technique. Previously, it was established that King was durable enough to withstand all of Zoro's blows without having to block them. Now, however, Zoro forced him to block all his attacks which goes to show that his offensive power was much higher at this point than it was before.
When the two clashed for the final time, Zoro's "King of Hell Three-Sword Serpent: 103 Mercies Dragon Damnation" was strong enough to cut through King's strongest attack and defeat him in a single blow. Overcoming a man worth 1.390 billion berries means that Zoro is much stronger than he was when the arc started. Enma became his biggest power-up in the arc and he has managed to make it his own over time. It wouldn't be a stretch to say that Zoro is now among the elites of the One Piece world, surpassing the level of the average Yonko Commanders.
Although he still cannot defeat the likes of the Yonko or the Admirals, Zoro is approaching that level fast and will eventually get there. The next step for him is to take on Dracule Mihawk and defeat him to declare himself as the strongest of all swordsmen. With Enma and the power of the King of Hell Three Sword Style, Zoro certainly has enough in his bag to give a great fight to Dracule Mihawk, if not defeat him.