Complex sentence: Domon learns that Schwarz had used Rain to teach him that he can't fight alone, and quickly finds out that Schwarz is his brother Kyoji when Schwarz's mask comes off.
Split into simpler sentences: Domon learns that Schwarz had used Rain to teach him that he can't fight alone. Domon then immediately learns that Schwarz is his brother Kyoji when Schwarz's mask comes off.