Beyond the Black River by Robert E. Howard
"Beyond the Black River" is one of the original short stories about Conan the Cimmerian, written by American author Robert E. Howard.
The foreword to the story tells of Conan's journey to Punt with Muriela, a scam perpetrated against worshippers of an ivory goddess, and then into Zembabwei, where he joins a trading caravan on its way to Shem. Around 40 now, Conan visits Cimmeria and finds his old friends are now fathers. Bored, Conan sets off for the Bossonian Marches and becomes a scout at Fort Tuscelan on the Black River, at the western border of the newly conquered Aquilonian province of Conajohara.
A young settler named Balthus encounters Conan in the forests slaying a marsh devil. 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 the fort is vastly undermanned after the Aquilonian king, Numedides, has foolishly decided to withdraw most of his army. Taking a hand picked team of scouts and Balthus, Conan sets off stealthily in canoes. Soon, Balthus is captured and most of Conan's men are slaughtered in an ambush. Balthus and one of the scouts are tied to stakes, while the scout is sacrificed by Zogar Sag to one of his jungle creatures. Before Balthus can meet a similar fate, Conan sets a Pictish village on fire and the two flee into the forest. Conan tells Balthus information on the cult of Jhebbal Sag, now forgotten by most. Once all living creatures worshipped him when men and beasts spoke the same language. Over time, both men and beasts forgot his worship. Zogar Sag has not, however, and can control those few animals and creatures who also remember, sending them on Conan's trail. Conan is able to neutralize the creature using a symbol he once noticed, and the pair return to the Fort to warn everyone of the impending Pictish assault, but they are too late. The Picts have already surrounded the fort, and a fierce battle is going on. The number of Picts ensures that eventually the fort will be overwhelmed and the defenders slaughtered. The only thing left to do is warn the settlers to flee while the Picts are busy with the fort - otherwise they will be slaughtered, too. The two soldiers go to warn the settlers that an army of Picts have crossed the river and are about to attack. They are joined by Slasher, a feral dog formerly owned by a settler who had been slain by the Picts. Balthus is sent on to warn th settlers of an incoming raid, and Conan parts from him to aid a group of fishermen who had gone to gather salt. Balthus warns the women and children to leave their huts and flee. When a band of Picts arrive, who move quicker and might overtake the women, Balthus stays behind to cover their escape. Accompanied by Slasher, he makes a stand against the coming Pictish raiders, first shooting arrows from concealment and then in a furious face-to-face battle. Both Balthus and Slasher's sacrifice delays the Picts while giving the settlers time to reach safety. Conan manages to warn the salt-gathering party in time, but finds he has been marked for death by the gods of darkness for misusing the symbol of Jhebbal Sag, and he is attacked by a demonic creature who tells him that it and Zogar Sag are of one blood.
Telling more would be quite a spoiler.
Abut 75 or 80 pages in print.