The Gryb — A.E. Van Vogt

Stranded in another universe, two mortals face a slow wasting - or a quick death from the gryb. A scale-armored, blood-sucker gryb glinted off the incredibly small and gem-like disk of sun, which rose higher and higher from the fantastic horizon of Europa, blinding Thomas. The man in front was desperately hard to see against the relentless brilliance, and it seemed as if he was deliberately holding himself in the sun's glare to distract Thomas's wearying mind and dull his strength. Had the guide uncovered the real reason for his mission to this alien world?