Moonstone

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A Moonstone is an artifact that is able to causes an eclipse that blocks out the sun and creates an artificial night, which then lasts for 30 seconds.

Source
These can be bought from Ancients of Wonders in Frozen Throne.

Malfurion, Tyrande, and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.