The Elfgates were a pair of magical gates created to protect the kingdom of Quel'thalas and Silvermoon City. During the Third War, Arthas Menethil invaded the high elves, the Ranger-General of Silvermoon,Sylvanas Windrunner, boasted to Arthas Menethil that the gates were imbued with potent magics. However, the undead Scourge led by Arthas slaughtered the high elf defenders and villagers and destroyed the first gate.
The second gate was an energy field made of magical energies known as Ban'dinoriel, the Gatekeeper, which could only be opened by the Key of the Three Moons, a magical item that had been split into three separate mooncrystals. Thanks to the traitor Dar'Khan Drathir, Arthas was told of the location of each of the three mooncrystals: Altar of Kings on intersections of ley lines at An'telas, An'daroth and An'owyn, heavily protected by high elf defenders. With the help of some goblin mercenaries, Arthas and his undead army destroyed the three bases and recovered the Key of the Three Moons. After that, they opened the second Elfgate and marched to Silvermoon City..
In World of Warcraft
The first Elfgate that was destroyed during Arthas' invasion could be the gate at Thalassian Pass, while the second Elfgate could have been located near Elrendar River, between Eversong Woods and Ghostlands.