WARNING: The following contains spoilers for My Hero Academia Chapter 335, "Zygotes” by Kohei Horikoshi, Caleb Cook, and John Hunt, available now in English from Viz Media.
All For One and Shigaraki may be My Hero Academia's main antagonists, but they were always supported by the League of Villains. The members of the villain group found themselves scattered during the Paranormal Liberation War and its aftermath, but most of them may be reunited very soon.
As All Might recounts Japan's biggest threats to Class 1-A, he mentions both Dabi, also known as Toya Todoroki and Himiko Toga, two of the League's very first members. Himiko split from the League in the middle of the Paranormal Liberation War to confront Ochaco Uraraka and Tsuyu Asui, but promised to return once she had found the information she needed.
Dabi, on the other hand, always made it a point to distance himself from the League. Even when the team was initially put together, he was always off on solo missions taking care of his own business. Unlike the others, he never readily joined in on the banter, preferring instead to keep to himself except for the occasional witty remark or comment. Following his epic reveal as Toya Todoroki, Dabi stated that he never cared for the League and was only using them as a means to his own end: Endeavor's destruction.
Dabi's return to Tomura Shigaraki's side must mean that the current Number One Hero is also in All For One's crosshairs. The only thing Dabi cares about is destroying Endeavor and everything he stands for. Allying himself with All For One must mean that they both share at least this one goal. Their partnership also raises the possibility that All For One may gift Dabi a Quirk that allows him to overcome the damage his own flames cause him. Dabi could become even more powerful than his last encounter with Shoto Todoroki and Endeavor.
The final League member confirmed to be with All For One is Spinner. Throughout the assault on Tartarus and everything that followed, he never left Tomura's side, even after he realized his leader had been corrupted by All For One. Currently, he forms the crux of All For One's plans for the future after completely destroying the heroes. Spinner has become the face of the Paranormal Liberation Front, a movement that is steadily gaining popularity, especially among those with heteromorph Quirks like him. All For One hopes to use him to manipulate the post-hero society, but Spinner's opinion on the matter hasn't been revealed yet.
Each member of the League is individually dangerous but they've proven time and again what a coordinated group they are. Now that they once again have All For One sponsoring them, the League has even more resources at its disposal. Their preferred mode of operation has always been covert infiltration with guerrilla-style battle plans. Their current target should be UA and the reveal of the traitor within the school walls will make their job that much easier.