Final answer:
The gothic elements, somber tapestries, ebony blackness of the floors, and the blood-red moon shining through a fissure in Zaroff's chateau contribute to an eerie atmosphere.
Step-by-step explanation:
One detail about Zaroff's chateau that provides an eerie or ominous mood is described in the passages that talk about the Gothic architecture, the dark and intricate passages, and the phantasmagoric armorial trophies which contribute to the uncanny atmosphere. The mention of the somber tapestries, the ebony blackness of the floors, and the house's overall gloomy and oppressive air as the character rode towards it all serve to create a chilling and foreboding mood. Furthermore, the description of the blood-red moon shining through a fissure provides an ominous visual that signals danger and dread.