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All answers having "Momentous" as clue:

4 Character Answers:

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EPIC



5 Character Answers:

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GRAVE

GREAT



7 Character Answers:

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DRASTIC

EPOCHAL

FATEFUL



8 Character Answers:

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EVENTFUL

HISTORIC



9 Character Answers:

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IMPORTANT



12 Character Answers:

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EARTHSHAKING



15 Character Answers:

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EARTHSHATTERING