In the My Hero Academia season 5 episode “That Which Is Inherited,” Izuku Midoriya (Justin Briner), better known by his hero name Deku, manifests Daigoro Banjo’s quirk Black Whip during combat training against Class 1-B, but how exactly did Deku inherit the new ability? When All Might (Christopher Sabat) explained back in season 1 that his quirk One For All is refined with each new generation of wielders, he had no idea when he transferred it over to Deku just how much the previous holders of One For All had strengthened the power with their individual quirks.
While Deku initially had trouble managing the power of One For All, he teaches himself how to control it and shapes the power in his own unique way throughout the first four seasons, developing Full Cowling and the close combat technique Air Force instead of copying All Might’s super strength. During a training session with Class 1-B, however, Deku is reverted to his early days of training when he gains a new quirk that uncontrollably exploded black cords out from his hand when he’s attempting to capture Monoma. After Shinso uses his quirk Brainwashing to calm him down, Deku has a vision of Banjo, a previous wielder of One For All, who explains that the predecessors’ individual quirk factors are mixed in with the core of One For All and that Deku will inherit all six of their quirks. However, the episode doesn’t directly address exactly how Deku inherited Black Whip from him or for what purpose.
The answer appears to be that the original wielder of One For All physically transferred Black Whip over to Deku. When All Might revealed the origins of One For All in season 2, he explained that the villain All For One gave his sickly brother a stockpiling quirk against his will, which activated a useless inert quirk he already had within him: the ability to pass on quirks. The abilities together make One For All, a quirk that accumulates strength with every new generation of users. In the season 5 episode “Vestiges,” the original wielder of One For All visits Deku in a dream and touches his hand, which then begins to glow red when Deku wakes up. Since Deku comes to the realization that the spirits of the previous users of One For All all live within the power, the original wielder used his quirk to transfer Black Whip - a quirk that emits black tendrils from the user's palms - into Deku’s hand when they touched in the dream.
Deku may have been the first wielder to inherit the multiple quirks within One For All because of the evolution of quirks in society. In episode 12, Deku suggests that there could be some kind of external connection when theorizing with Bakugo and All Might about what might've triggered Black Whip to manifest. That connection could be the Quirk Singularity - an in-universe theory that suggests quirks will only get stronger with each new generation and evolve to a point when they’ll become unmanageable. In episode 10, Gran Torino tells All Might that Nana Shimura had a dream similar to Deku’s, in which a shadowy figure appeared to her and told her, “The time has not yet come.” When the original wielder appeared to Deku in his dream, he informed Deku that “we’re long past the singularity point.” With this in mind, the quirks in One For All are emerging now because Deku will need extra power to combat the oncoming dangers posed by the Quirk Singularity.
By having the original wielder begin to pass down the quirks inside of One For All to Deku within season 5 of My Hero Academia, it creates an interesting juxtaposition between All For One and his brother. While All for One used his quirk to distribute powers in order to gain followers and support his own agenda, his brother can now pass on six new powers to Deku that will benefit the greater good. Since All For One may have contributed to the escalation of the Quirk Singularity by passing on quirks to others that evolved over time, the original wielder now has the chance to correct his brother’s misdeeds through Deku.