What Comes Suddenly and Never Leaves?

Riddle Story of Devil — Episode 4: "What Comes Suddenly and Never Leaves?"

Ep 1 The World is Full of ___ Ep 2 What Dwells In Your Heart? Ep 3 What's Red, But Isn't Red? Ep 4 What Comes Suddenly and Never Leaves? Ep 5 How Do You Get A Bird Out of Its Cage? Ep 6 Every Pretty Flower Has Its ___ Ep 7 What Can You Never Outrun? Ep 8 Which Gatekeeper Is the One Who Lies? Ep 9 What Dwells In Your Heart? (Make-up Test) Ep 10 Who Is the Queen? Ep 11 How to Tell Apart a 'Felicitation' From a 'Curse' Ep 12 Therefore, the World is Full of ___