In the the episode 10 of the 1st season of My Hero Academia, the music starts around 15:00 (more precisely at 14:53). It is also available in episode 9 of season 3 at 15:47.
What is this song?
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The name of this OST is Mind of Evil. This OST is by Yuki HAYASHI and can be found in the track 27 (YouTube link) of My Hero Academia Original Soundtrack.