Answer:
inveigh
Step-by-step explanation:
Interloper: a person who's involved in a certain event, but they don't belong there and aren't wanted
Millennium: a period of 1000 years passing
Lassitude: state of lack of energy; languor
Inveigh: to speak with hostility; to attack in words
Intrinsic: naturally essential
Therefore, the answer is inveigh.
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