i think it's a one shot where the light prince's sister have to marry the neighbouring country's king, but she doesn't want to and claimed she like someone else. maybe she just doesn't want to marry the king as his castle is always in the darkness. she is nice to the light prince and he took her place and went to the dark king's castle. then the king decided to let the prince return to his country but he doesn't want to as the prince said that even though there's light, he's always lonely so he would rather stay with the king.

if anyone know the title, please help 🙂

I think what you're looking for is Kizoku no Aoi Seppun Chapter 5: The Kiss that Melts Darkness

There's also another story that involves light and dark called Neo Arcadia - Yami ni Saku Niji. You should check it out if you're interested or haven't read it already.

I LOVE YOU! Thanks <3 It's been bugging me for a couple of days now and i could only remember the word darkness from the title ._.