Horn of Cenarius

The Horn of Cenarius was the horn of none other than the demigod Cenarius. After his death, it remained on Moonglade Isle and was guarded by the Primal Guardians. It was retrieved by Tyrande and used to awaken Malfurion Stormrage, and then later used by Malfurion to awaken both the Druids of the Talon and Claw. Following the Battle of Mount Hyjal, the Arch-Druid blew the Horn again, summoning the power of nature to destroy Archimonde.

History
The horn was originally used by the demigod Cenarius to call upon the powers of nature. Since druids are in tune with nature and represent it, the horn also effectively works as a wake-up call to them.

According to legend, this powerful item had the power to call upon the spirits of all night elves. Although Cenarius normally kept his horn for himself, he loaned it to others on occasion. According to tales, the horn has other powers besides awakening the night elves. A rumor hints that the horn's bearer will gradually be turned into a night elf if he carries it for long, but as only night elves have possessed this item for any length of time, this legend cannot be substantiated.

When Cenarius perished, the location of his magical horn was lost from the ken of the mortal races. Although many have searched for it, none had discovered its location. Rumors suggested that the forest itself claimed this lost artifact for all time, and yet it one day may again appear when the need for it is great enough It was later recovered by Tyrande to be used during the Third War.

Source
It was gained by Tyrande in The Awakening of Stormrage, and used to awaken Malfurion. Malfurion uses this in The Druids Arise, Brothers in Blood, and Twilight of the Gods which are chapters 4, 5, and 7 of the Warcraft III night elf campaign, Eternity's End.