The Thing with a Thousand Heads (Shadowdark)

From publisher blurb:

A crumbling fortress in the plains of Kush is avoided by wildlife and sensible travelers – an ancient evil aura emanates from within its lone standing structure, a two-floor tower overlooking the east. Western sections of the fortress lie in ruins. Lions and tigers will stop within 100 feet of the building, terminating their pursuit of prey. Even the grass itself withers near the base of the tower, creating an unmistakable line of demarcation surrounding its base. Only the most desperate of travelers, trapped in the region’s frequent lightning storms and torrential rains, have dared to seek shelter in this deadly place—the final decision they would ever make.