AFO tells Deku that while he may have taken One For All, and by extension, Yoichi, from him, the fact that the young hero attacked Shigaraki mentally left him vulnerable enough for him to finally take over! Indeed, Shigaraki begins to vanish from the mental world as AFO reveals that no decision his protege made in his entire life was ever his own.... he was simply manipulated into believing that he had his own will.
We learn that after failing to capture OFA from two of its previous users (Banjo and En), AFO realized that what he needed to control it was an iron will.... so he set his plan into motion after All Might came to power. He met Shimura Kotarou, and convinced him to have another child since his firstborn (Tenko's big sister) wouldn't have been able to handle the transfer. After Tenko's birth,
AFO took his original quirk, and replaced it with another! The replacement was a quirk that was copied by Dr. Garaki, known as
Disintegrate. It was originally capable of both decaying AND reconstruction (I guess sorta similar to Overhaul's quirk), but it was modified so that only the destructive element remained. So yes, Tomura has literally been AFO's puppet his entire life.....
After revealing his master plan, AFO ejects Deku from his mind, then erases Shigaraki, leaving him in complete control! Back in the real world, we see Deku is missing both his arms. As for ShigAFO, he has a hand covering his mouth now, and actually notes that his possession is still imperfect, due to all of the prior damage Shigaraki suffered, both physically and mentally. Regardless, he still aims to conquer the world! He prepares to finish off the helpless Deku, when suddenly he gets stopped by....
Sero! And not just him, both Ojirou and Satou arrive to provide backup, as well! The last to appear is Aizawa, who apologizes to Deku for being late as he steps out of a portal....!!!!