In the world of My Hero Academia, created by Kohei Horikoshi, All Might has always been an integral part of the story. Introduced in the world of My Hero Academia as the greatest Hero to have ever lived and Japan's number One Hero, All Might was the idol that everyone looked up to and the Symbol Of Peace that society held on to tightly.
Kids, such as Bakugo, Shoto, and, of course, the protagonist of the story, Izuku, looked up to All Might while growing up and idolized him. He was the reason why a generation of Hero aspirants joined the prestigious Hero Academies in an attempt to become a Hero and he became the spark for everyone's beloved protagonist to set out on his journey. However, as the story progressed, All Might's journey itself became quite troublesome. He ended up losing the vast majority of his powers, and, eventually, all of them. Crippled and, for the very first time, in need of protection himself, All Might became a shadow of his former self. Yet, now, as revealed in the events of My Hero Academia chapter 386, All Might is back once again.
How All Might Lost His Power
All Might was known to be an incredibly powerful Hero, worthy of the Number One ranking at the beginning of the story. However, it wasn't much later that it was revealed to the fans that All Might was actually running very low on power, especially after All For One injured him during their fight which he ended up winning. Due to his severe injury, All Might was no longer able to fight for prolonged periods of time at his very best. For this reason, he started looking for a successor and initially set his eyes on Mirio. Yet, through a twist of fate, Deku proved his selfless Hero nature and became the next user of the One For All.
When All Might passed on the One For All, whisps of this power remained in his body and he ended up burning all of them up in Kamino, where he fought All For One, for what appeared to be the last time. Standing tall as the greatest Hero of all time, All Might pushed beyond all his limits and ended the tyrannical rule of All For One, sending him to Tartarus in the process. In doing so, his era finally came to an end and he lost all his powers, Which meant that Deku now had to carry the Hero Society on his back.
All Might In The Final War
Following the loss of his powers, All Might became crippled to a point where he couldn't even fight the weakest of the villains. His level of strength was not even remotely close to what it was before and even his stamina had suffered terribly. His injury held him back from doing the most basic of combat-related activities and it was quite clear to see that he could not play the role of a Hero again. Endeavor, the Number Two Hero before him, ended up rising to the Number One position and attempted to fill the boots of All Might. While he did reasonably well, he was no All Might.
For All Might himself, the feelings of helplessness became more apparent with time and for someone who has been lifting the entirety of the Hero Society on his back and saving everyone with a smile, to be this helpless was certainly a crushing blow. All he could do was entrust everything to the next generation of Heroes and hope for the very best, which is what he did for the vast majority of the story from when he lost his powers to the Final War Saga. Even when All For One and Shigaraki were running riot on the battlefield and Heroes were dying left, right, and center, All Might could not do much other than observe. However, this changed towards the climax of the arc where he finally set out into action.
All Might's New Power
Upon hearing that All For One was getting stronger and had even surpassed his prime self, All Might couldn't just sit back and watch. He was seen ordering Shoto and Iida to pursue Dabi and to leave All For One to him. Even though he had no Quirk, he set out to fight what is the strongest form of All For One and he is doing so with huge confidence in his abilities. In the end, Deku, who was once a Quirkless boy, ended up being his Hero. In his Quirkless state, Toshinori could only look up to the boy who broke into tears on that fateful day and asked him if he could be a Hero.
Posing the same question to himself, All Might set out to fight against All For One. While fans might think that he's going to get manhandled in this fight, or worse, even die, it must be remembered that All Might is not empty-handed. He seems to be carrying some sort of Hero Equipment with him. This equipment would likely have been built by Mei Hatsume, and if one had to guess, this could be a full-body suit created so that All Might could come back to a reasonable level of strength. From what fans saw in the previous chapter, Toshinori carried a briefcase with him which he broke as soon as he saw All For Ond. It is possible that inside this briefcase is a suit of armor that would encase his body and allow him to fight against All For One to the fullest. Either way, All Might is not heading into battle helpless and defenseless. He has a power-up that he intends to use and, given that he too, like Deku, wants to change destiny for the good, All Might will not die and do more than enough to hold All For One at bay.