Tharun is a hollow world with an island realm modeled after feudal medieval Japan. It was specifically designed for German RPG DSA-Professional, setting itself apart from the established setting of Das Schwarze Auge, Aventurien.
The adventures published in the two rules box sets intended to have the players take over the island they arrive on, then expanding their influence on the group of islands in the vicinity and thus conquering the ruling hierarchy. Since DSA-Professional and Tharun were discontinued after the release of said two box sets, no further exploration of this world was published until 2014.
Later editions of Das Schwarze Auge, which established Dere/Ethra as one of many layered planes of existence as opposed to a planet in a solar system (and thus containing the hollow world of DSA-Professional) redefined Tharun as a magical globule: a self-contained magical realm parallel to the main cosmology.