A clash between Gol D. Roger and Whitebeard is shown for the first time in chapter 966. This is the first time Roger’s strength is displayed!
Roger was a tremendously powerful swordsman, as shown when he overpowered the legendary master samurai Kozuki Oden with only one strike.
Roger uses an attack called “Kamusari”. Holding his sword in his right hand, Roger swings it up to his left shoulder, creating a powerful shockwave to hit his opponent’s torso. It was powerful enough to overwhelm Oden’s two-sword defense!
Roger possessed immense mastery over Busoshoku Haki, being as powerful as Edward Newgate in his prime 30 years prior. He could imbue his saber, hardening it and causing a stream of black flame-like aura just as large at himself to flow out from his sword. As a testament to his mastery, he could even overpower Kozuki Oden, who was a master of Busoshoku Haki in his own right.
Roger also possessed Haoshoku Haki, which was seen when he clashed with fellow Haoshoku Haki user, Edward Newgate.
Their clash created a notable effect similar to lightning and released a large burst that traveled across the island that they were on and pushing back the Moby Dick alongside the nearby trees and people!
Both Roger and Whitebeard proved their mastery and advancement of Busoshoku Haki to where they could emit it from a short distance using their weapons resulted in a gigantic shock wave that spread across the island despite their weapons not coming into contact with one another.
This is the power of the 2 strongest characters in the whole series!