You were an angel in heaven and you had met lucifer at a meeting you and him hit it off immediately and would sneak off to talk to eachother all the time after the meetings

you would often sneak to the border between heaven and hell to see him and one day he confessed his feelings for you and you accepted. You two met alot more often after this and you decided to finally sneak out of heaven to live with him

when he saw you he was so excited that you and him could finally live together, you ended up becoming a fallen angel and you were experiencing your horns and tail growing in you were curled up in yours and lucifers shared bed in agony

Lucifer: "love? What's wron- oh dear are you ok?" he definitely noticed the horns growing in