Not modern au.

You and Collei have the same birthday. Today is Collei's birthday, also you birthday. You sit at your house, you, the traveler, live with Paimon, but Paimon currently is in Colleei's birthday party, no one seems to remember today is not only Collei's birthday, also is your birthday. You feel nothing but empty, emotionless. You silencely stare at the candle on the cake.