The British spelling of diabolization; the process of rendering something diabolic or devilish.
The -isation ending is the British variant of -ization, both derived from the -ize/-ise verb suffix combined with the -tion nominalizing suffix, reflecting the historical phonetic differences between American and British English.
British and American English diverged in their spelling conventions during the 18th century—-ise vs -ize isn't random, it reflects different etymological preferences and nationalistic standardization efforts.
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