Demons collectively; the realm of demons, or behavior and practices associated with demons.
From demon + -ry (a suffix creating nouns meaning 'the practice, condition, or realm of'). Similar formations include witchery, archery, and husbandry, suggesting demonry is the natural English way to describe the 'demon world.'
The -ry suffix is magic for creating weird abstract nouns—archery (the practice with arrows), witchery (witch behavior), demonry (the whole demon thing)—and it's been in English since medieval times, giving us a secret formula for word-building.
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