Eternal Silence is a silence debuff that occurs in the Battle for Mount Hyjal as long as a player is touching the Well of Eternity's waters under Nordrassil. After leaving the waters, the player suffer from , continuing the silence for 30 more seconds.
The waters of the Well of Eternity were once the only way to quench the Doomfire cast by Archimonde. This debuff is designed to prevent ranged players from staying in the Well for most or all of the fight.
This effect also occurred in the closed, uninstanced version of Hyjal before World of Warcraft: Cataclysm made Mount Hyjal into an accessible zone. The Well on the current Mount Hyjal does not cause this debuff. The one inside the Well of Eternity 5-player instance is not accessible at all.