Robert E. Howard 
Beyond The Black River (Illustrated) [EPUB ebook] 

The foreword to the story tells of his travels to Punt with Muriela, refers to a scam perpetrated against worshippers of an ivory goddess and then on to Zembabwei, where he joins a trading caravan on its way to Shem. Around 40 now, Conan visits his homeland and finds his old friends are fathers. Bored, Conan sets off for the Bossonian Marches and becomes a Scout at Fort Tuscelan on the Black River. Naturally, there is a war going on…
A young settler named Balthus encounters Conan in the forests slaying a forest devil. Accompanying the young man back to the Fort, Conan finds the body of a merchant ensorcelled by a Pictish wizard named Zogar Sag and slain by a swamp demon.
The Fort Tuscelan Commander, Valannus, is a desperate man and asks Conan to slay Zogar Sag before he raises the Picts against the whole borderlands. Taking a hand picked team of scouts and Balthus, Conan sets off stealthily in canoes. Balthus is captured and most of Conan’s men slaughtered in an ambush.
Balthus and one of the Scouts are tied to stakes and the scout is sacrificed by Zogar Sag to one of his jungle creatures. Before Balthus can meet a similar fate, Conan sets the Pictish village on fire and the two flee into the woods. Conan tells Balthus of the cult of Jhebbal Sag, now forgotten by most. Once all living things worshipped him when men and beasts spoke the same language. Over time men and most beasts forgot his worship. Zogar Sag has not, however, and can control those few animals and creatures who also remember. And they are on Conan’s trail now.