Sono Yo no Dokoka, Tokoyo no Rakuen

On hearing some concerning news, the adventurous Prince Shin of the great kingdom of Garche infiltrates the desert kingdom of Tarchia. There, he meets Yashum, a man running a private school. While he seems shady at first, Yashum is incredibly knowledgeable, and he's accompanied by the youth Amaru, who's so fond of his teacher that he's as protective of him as a knight! However, in this small kingdom with a tragic history, a drama is unfolding between this odd pair over a "mystery drug..." A maddening, codependent love blossoms in a paradise where dawn never comes.

(Source: Renta!)

Note: Includes an extra chapter.