Robert E. Howard 
Beyond the Black River [EPUB ebook] 

The story takes place in Conajohara, a newly established Aquilonian province recently annexed by King Numedides from the Picts. Balthus, a young settler on his way to Fort Tuscelan at the Black River, the province's border to the Pict Lands, encounters Conan in the forest slaying a Pict. Accompanying the young man back to the fort, Conan finds the corpse of a merchant left by a Pictish wizard named Zogar Sag and slain by a swamp demon. The fort's commander, Valannus, desperately asks Conan to slay Zogar Sag before he raises the Picts against the whole borderlands, especially since Tuscelan is vastly undermanned after Numedides foolishly decided to withdraw most of its garrison. Taking a hand-picked team of scouts and Balthus, Conan sets off stealthily in his canoe.