Of or relating to demons; demonic or characteristic of demons.
From demon + -ical (adjective suffix). An older variant of 'demonic' using the -ical ending instead of -ic.
The -ical ending (like in 'magical,' 'cynical,' 'mythical') often sounds more formal and archaic, which is probably why 'demonical' appears mostly in older literature and religious texts trying to sound suitably grave.
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