Hello! I'm new in this forum and I'm a bit desperate to find this manga as I have been trying to remember it for months now and it's wrecking my brain. I only remember parts of it since i last read it was 2011 so I hope it helps.

Genre: either Yaoi or Shounen-ai but im sure its BL
Period: historical around time where there were kings and concubines and stuff
Plotline: Main character assassin tries to kill the King who's asleep on his bed chamber but instead was caught by him so the assassin turned his knife to himself and tries to kill himself since he failed his mission but the King stops the assassin and had sex with him instead. the king then advices the assassin to pretend to be his concubine so the assassin can try at every opporunity to kill him again (i cant remember the middle part) so i think he accepts the offer to pretend to be his concubine so he can try to assassinate the king again but fails everytime and every failure means sex for the king. he dresses up as a female concubine all pretty and some of the king's concubines are jealous of him. then one day another assassin was sent and the main character assassin fights it off with the 2nd assassin. (i also cant remember the ending.) hahahaa! well. thats all i can give you.

and ohh.. i'm not sure but i think this is a oneshot? )

Sorry, can't help you but the story sounds amazing o.o *wants to read it too*

Hello Welcome to the forums.
The manga is chapter 2 Prince vs assassin from Oujisama game.

HAHAHA! @KaintheVampire yeah, story was soo good that i still remember it now. Just forgot the title. Now we can read it Thanks to @Zephyr4! Thank you so much! you just gave me the best christmas and new year's gift EVER!!

Glad to be of help and have a lovely 2018!