This is a scene from the last chapter of "Villains Anonymous". Tirek just got caught stealing Derpy's magic and decides to spill the beans as to why he's so evil. In a fit of rage, he tells them that it's because he hates his family so much and wants revenge on his brother Scorpan for leaving him in Tartarus. This scene was rather emotional and definitely brought out a darker side of Tirek that was more serious than anything they ever showed with him in the show itself. After Discord said something about Vorak in the finale, I knew it all had something to do with Tirek's family.