〜なかれ
do not (classical command)
Verb dictionary form + なかれ
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なかれ is a classical negative command, "do not", found in mottos, dramatic writing and set phrases: 失敗を恐れるなかれ, "do not fear failure".
It follows the dictionary form, like the plain prohibitive な (N4's imperative): 恐れるな. なかれ sounds lofty, like "thou shalt not".
驚くなかれ is a lively way to introduce surprising news, "believe it or not": 驚くなかれ、彼はまだ十歳だ, "believe it or not, he's only ten".
事なかれ主義 is a common noun meaning a "don't rock the boat" attitude, avoiding trouble at all costs. In everyday speech, you'd simply say 〜な or 〜ないで.
Examples
驚くなかれ、彼はまだ十歳だ。
Believe it or not, he's only ten.
あの会社は事なかれ主義だ。
That company has a don't-rock-the-boat attitude.
失敗を恐れるなかれ。
Do not fear failure.
汝、嘘をつくなかれ。
Thou shalt not lie.
最後まで油断するなかれ。
Don't let your guard down until the very end.