Oh, the possibilities...This is a good storyline. I will think about it=D.

A tale of two lovers who were never fated to be together.

In their first life, Sehun is working for the King as his bodyguard; Luhan is a rebel who's plotting an assassination of the King. The ploy fails and Luhan falls into the hand of the King. Seeing a resemblance between Luhan and his late younger brother, Sehun manage to persuade the King to allow him to handle his punishment. By the works of fate, the two slowly fall in love. However, Luhan is still bent on killing the King. The second attempt failed miserably and Sehun is pressured by the King to kill Luhan. Unable to execute his beloved, Sehun turns the sword and impale himself with the sword instead. Insert your first paragraph here. Luhan and Sehun dies and the screen fade to black.

When Sehun wakes up, he finds himself in a young boy's body and realized the Cupid have seen his act of true love and decided to give him a second chance to live. However, with this body, he could only live till he's 10y.o. In order to break this limit, he have to search for Luhan - who is a bounty hunter who is well known for the skill of killing people by smearing a highly toxic poison on his lips and biting their lower lips to allow the venom to be injected into their bloodstream. Death occurs 10 seconds after the venom comes into contact with fresh blood. This technique of killing is hence coined the "Kiss of Death". Unbeknownst to Sehun, Luhan's next target is him.

uuuuuu....

hmmmm
maybe...
they were like childhood friend that have super power, but then they got separated because of different powers
they were (something anything brain wash or something) and they unfortunately forget each other, and Luhan become the dark lord or evil magician and Sehun become this hero that same the world and protect the human kind with the others
Luhan and his underlings was trying to destroy the human kind but Sehun and her team mates protect the human kind
after so many failing Luhan kidnap Sehun team mates and Sehun have to save his team mates! Luhan leave a message and they suddenly fall in love and when Sehun come to the cave of darkness he met Luhan and just know that he is the one for him and they die in each other hand

or!

they meet as and become friends but they don't know that each other were against, Luhan is evil and Sehun is the good guy. Luhan always fails to do evil things because of sehun. they both fall in love and in there they know each others secret secretly, so that Luhan knows about Sehun from others and Sehun know about Luhan from others
they wanna fight about it but they know that they like each other so they die in each others arms

if it was me, i would write this during the greek ruling eras. nd luhan is the god of war, ares when he came to earth. sehun is a young greek soldier. he's the son of morpheus x human woman or sth and he did a bad thing nd ares wanted to punish him (or kill him). idk, thats my idea ^^

Well what you wrote can be an ending of a battle between good and evil? Life and death?
Or maybe this will be just the begining, and they could be sent in a dark universe or something and they have to find each other again :D