At the end of chapter 348, Deku left the battle with Toga to head towards the fight with Shigaraki.

Source: https://screenrant.com/hero-academia-deku-abandon-good-chapter-348-explained/

Deku's new dark appearance will be explained later this season, with the effects of the fight against Shigaraki clearly having a major influence on what the wielder of One For All is ready and willing to do to save lives.

Source: https://comicbook.com/anime/news/my-hero-academia-season-6-new-op-dark-deku-watch/

Despite raising Shigaraki since childhood, All For One's sole objective is to claim his body for personal use.

Source: https://www.cbr.com/anime-villains-fans-love-hating/

During the Paranormal Liberation War arc, Deku was finally able to tap into its power when Shigaraki unleashed his Decay Quirk to destroy everyone around him.

Source: https://gamerant.com/my-hero-academia-one-for-all-quirks/

If you have not been keeping up with the series, you should know the final act has kickstarted its big war against All For One and Shigaraki.

Source: https://comicbook.com/anime/news/my-hero-academia-manga-anime-canon-cameo/

Now that the fight is over, it's finally time to start focusing on the true final battle in the manga as Deku fights Shigaraki.

Source: https://comicbook.com/anime/news/my-hero-academia-manga-deku-quirks-stolen/

Though he speaks with Tomura Shigaraki through a radio transmission during the League's multiple schemes, he is formally introduced much later in the series.

Source: https://gamerant.com/my-hero-academia-villains-best-introductions-ranked/

When Shigaraki first arrived at U.A, he was unprepared and inexperienced.

Source: https://www.cbr.com/league-of-villains-rescued-each-other/