Magia Ex Machina
Eons ago humankind walked among the stars. Magic and technology allowed them to spread from one world to another. They met other races, fought wars and built great works. The age of star walking lasted countless centuries, and there was almost no world that did not feel the tread of sentient feet.
All things must come to an end though, and one day eight thousand years ago a terrible thing happened. A mighty spell was wrought by a civilisation now lost to history. Sorely pressed by their enemies, the mages of this civilisation created a process by which the spaces between the worlds would be filled with awful things bought from some distant, terrible dimension. The horrors filled the void between the stars, attacking all who sought to pass by whatever method.
For countless centuries interstellar travel was impossible. In a few cases it was maintained via magical gateways, specially warded or patrolled corridors or the like. Most worlds however were isolated beyond the veil of the horrors, and forced to develop on their own.
Now however, the spell has been unmade. Weakened by a thousand years of passing time, the great matrix which allowed the monsters to remain has been destroyed by some vast counter magic, dispelling most and leaving only a few to terrorise sapient beings. The skies are open once again, and now it is a matter of discovering what lies on the other side of them.