Summary...
Natsume and Ruka was born as "Fates", Fates are the ones who tend to break human's fate.. they keep humans in making their destiny happen, that's their mission, There is only one rule they must abide.. never fall in love with humans..
They created a mistake, they were thrown down to the earth... that was their punishment.. being sent down the earth.. and breaking one's fate... simple but.. what if in their next mission their task was to kill someone... not just someone... it's the person they fell in love with in human world.