Moonstone

Were you looking for the World of Warcraft item ?

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