The Lightless Dome — Douglas Hill

In the shadowed swamps far from Prince Phaedran's fabulous city of Quamarr, a sorcerer tests the fabric of the universe. At his command something stirs in the emptiness beyond the stars. It is incalculably evil, immeasurably old. It is summoned by the promise of fresh human souls...

Meanwhile in a 20th Century film studio, Red Cordell takes another cheap part in another cheap swords and sorcery movie. A beautiful neglected sword he finds in Props is his only satisfaction.

But in Quamarr, the enchantress Aurilia is in mortal danger. When she calls on the sword of Corodel, Red finds himself at her side, fighting for her life in a land of waking dreams...