I haven't read Conan the Great but I liked the idea of the keep, ruin and demon so I created a scenario of my own with them.
Here the gist if it. Feel free to use it for ideas:
The Demon in the Mist
The PCS are mercenaries serving in the Corinthian army. Until recently, they had an easy life performing garrison duty in the warm streets of Polliponi.
Now, their officer has assigned them as reinforcements to a keep on the mountainous border with Ophir. The commander of the keep has requested some more men.
The PCs travel along the cold, windy trails to the keep, fighting off an attack by bandits.
Arriving at the keep, they find an air of gloom and fear hovering over the men. The commander admits that 8 of his men have disappeared. Two disappeared first, then the search party sent to find them vanished and one man was found raving about monsters before he died.
That night, a mist falls across the keep and screams ring out as another another man disapears. The PCs catch a glimpse of a shadowy form with many tentacles and bat like wings.
Interrogating the other soldiers reveals that the first two men had deserted to do some treasure hunting in a temple in the mountains.
The commander orders the PCS to investigate the temple. En route, they meet some local Hill-men. They also are searching for a missing comrade. They tell the PCs the legend of the temple, how a dark cult occupied it serving the demon Kthantos, until the sorcerror Zathron destroyed them and bound the demon in the temple with a magic sigil.
The missing soldiers have released the demon and are now act as his possessed servants. They attack the PCs as they approach the temple.
"Like fiends from some darksome pit they seemed, lumbering forward, their red eyes glowing in the darkness.......... "
In the misty temple the PCs find the door with its imprisoning symbol shattered. Inside the demon lurks. Attempts to kill it fail unless the party scholar (or Hill-shaman) can paint the symbol on a weapon and drive it into the monsters heart !!!!!!!!!!!!