Nightmare Fortress ~ Majin no Jousai

Main storyline: There is a kingdom on a island somewhere in the four seas (sry- not too fluent in Japanese), all was well until oneday it was attacked by an army of Demons via. sodomy (fill in your imagination)- and of course the Prince of this kingdom was taken prisoner by the Devil King. With his last strength*cough* the Prince casts a spell that takes this island to another realem/make invisible- so that this "carnage" is maintained. Our main 2 characters are the Princes #1 bodyguard, and the bodyguard's protege- so thus begins their quest to regain the kingdom, kill the demons, and save the Prince. But it won't be that easy, cause watch out the Demons are always ready for a through tumble!

(Source: Manga Updates)