Nightmare Fortress ~ Majin no Jousai

Main storyline: There is a kingdom on an island somewhere in the vast seas. All was peaceful until one day it was attacked by an army of demons through some dark and unimaginable means, and the Prince of this kingdom was taken prisoner by the Devil King. With his last strength, the Prince casts a spell that transports the island to another realm or makes it invisible, ensuring this "carnage" remains contained. Our main characters are the Prince's head bodyguard and the bodyguard's protege. Thus begins their quest to reclaim the kingdom, defeat the demons, and rescue the Prince. But it won't be that easy, as the demons are always ready for a fierce battle! (Source: Manga Updates)