“Sillenian” means “those who worship Sillevan”. Sillevan was a man, more or less.
In this land there is a legend that says that more than one thousand years ago the people were living in another land very far away, a very old one, corrupted kingdoms and cities, everything full of wars and chaos. The god called “Gah” (also known as St. Cuthbert in the D&D Manuals) sent his son Sillevan between the Men in that land far away. Sillevan spread the word of Gah in those old kingdoms and told the Men that those lands were about to be destroyed because of their corruption. He begun to build a great Ark to save the people who believed him. The day before the fearsome cataclysm that destroyed everything there, all the people that believed in Gah and followed Sillevan entered this Ark and escaped from there to the New World, also called the Orb. Fare well to the old world.
The Ark traveled a lot of time, says the legend, maybe months. There were thousands of people inside. How? Nobody knows, it is a legend. Few of them started to think that they had been fooled by Sillevan and tried to go back, but this was not possible. So things went worse and eventually it was a battle inside the Ark between the True believers and those who didn’t wanted to follow Sillevan anymore, most of them Wizards.
The Wizards captured Sillevan in the struggle and killed him in the Ark’s keel.
The day after that, the Ark arrived the New World. This fact gave a lot of strength to the True, and them attacked the Wizards and killed all of them but one. The one that escaped use to be called “the Invisible”, and nowadays this word is synonymous to “devil”. And this is why magic has never been very good thing here.
From that day to nowadays, the people that came in the Ark are called “arkans”. This is the name of the main race in these lands.
So the arkans were in that New World. With the remaining of the Ark they built a church devoted to Gah, Sillevan’s Father (also called St. Cuthbert) God of Justice and Law. His holy sign is like a cross that represents the Ark’s keel where Sillevan died. Then they built houses and walls around the church, and founded a city. They called it Akenar.
They were not alone in those lands. There were other kingdoms, other people there. But that is another story.