Between Worlds

The Inbetween is the space between worlds.
 Worlds sometimes bridge to oneanother and when they do they pass through the Inbetween. For the Inbetween is between all. 

Things that fall Inbetween will drift until time immemorial but a certain chunk of rock has become a resting place for travelers that brave the Inbetween. Adventurers and Traders come here from various worlds and exchange their ways of life. There is a tribe of Vikings guided here by their Serpent deity, there are the Robots who eagerly study the mysteries of the Inbetween hoping to harness it for power, and there's the Watchers, floating eye-ball creatures that can't communicate as we do but they have mysterious powers of their own and are ever watchful of all the passerbys Inbetween. 

The Island Inbetween houses an ancient ruin which predades all the races. No one knows what's inside for it is locked by a mysterious force. Many think it must have been built by an advanced ancient civilization which could bend the Inbetween's mysterious properties to their own will. Opening this door could reveal the secrets of a bygone age, technology to wield the Inbetween or show the final resting place of a once great civilization. Perhaps the 3 races have the means to help you open this door.