Sabine Cheng a gaurdian angel was pregnant with twins.When she gave birth something strange happened.The girl was an angel but the boy was a demon.So the boy had to be sent to hell.
When Marinette discovers her twin is in hell she travel there to get him.To find him she needs the Prince of hells help.
Damian a usually cold and ruthless person has a soft spot for the angel infront of him.He helps her and they both end up falling for each other.