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All answers having "Fiendish" as clue:

4 Character Answers:

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EVIL



6 Character Answers:

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LACKEY



7 Character Answers:

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DEMONIC

SATANIC

SCATTER



8 Character Answers:

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DIABOLIC

INFERNAL



9 Character Answers:

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DEMONICAL

SATANICAL



10 Character Answers:

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DEMONIACAL

DIABOLICAL



11 Character Answers:

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FINISHEDOFF