When you determine something. You'll make it happened. This is the scene where they finally get to see each other. Cozy nearly destroy the magic permanently. Cozy thought so that equestria is defeated. She ask Lya to rule by her side. But Lya did not say refuse nor choose to ran away. But rather, she hug her "I love you. I always love you. Please.. Return back to me". Cozy finally realize her mistake and she give up her tyranny and throw it away. The magic is restored. Twilight wanted to turn her into stone but Lya beg her not to. Twilight don't have any other ideas but determine she already made "Lya.. I know how pain this is". Lya cried and angrily sad "No you don't!". Twilight sad as well because how much she love her pupil "Lya.. She did this before. I don't want to put my friends and equestria in danger. I'll.. Give you one day to spent with your sister". They both start bonding before she turned. Lya visit her everyday and talk to her. Cozy glow cried in that stone prison and Lya sees it "I promise". Twilight look at Lya how sad she is and doubted herself about love. So she decided to freed her. With haste they both hug with forgiveness and cozy glow said "I promise I won't leave you Lya"