Ban'dinoriel

Ban'dinoriel (Thalassian for "the Gatekeeper" or the Gate Keeper) is the name given to the nigh impenetrable shield that once surrounded Silvermoon City and the rest of the elven kingdom, forming a component of the second Elfgate. The dome was empowered by the Sunwell, and maintained passively via an intersection of ley lines stemming from three ancient mooncrystals at An'owyn, An'daroth, and An'telas. The three locations were hidden in an invisibility stasis, to keep their locations a secret from invaders. While the dome could be maintained without the crystals, it would require the Magisters to constantly empower it themselves.

Ban'dinoriel was erected around 6,800 years BDP to prevent others from detecting the elves' usage of arcane magic as well as to frighten away the superstitious Amani trolls. It proved a constant thorn for the Amani trolls, whose campaigns had never managed to overcome it, nor find the source of its power. Likewise, the Old Horde assault could not pierce the shield, and Silvermoon remained protected throughout the Second War despite the orcish necromancers' best efforts to subvert the shield. This changed during the Third War, in which the treacherous magister Dar'Khan Drathir, under the guise of wanting to ensure there were no weak spots in the land's defenses, used his friendship with the ranger lord Lor'themar Theron to learn of the mooncrystals' locations. Drathir sold this information to Arthas Menethil, who acquired them during the Scourge invasion and killed their defenders. Dar'Khan, knowing the magi could maintain the shield in person, murdered as many as he could find in the Sunwell Plateau, and performed a binding ritual to lock the elves out of the Sunwell's power, dispelling Ban'dinoriel in the process.

Revitalization?
With the restoration of the Sunwell, it may be that Ban'dinoriel's protection could be re-implemented by the blood elves. While the mooncrystals are presumably gone, it was mentioned that the magi could maintain the shield themselves in their place.