Empower Rune Weapon ("ERW") is a death knight ability learned at level 76. It allows the death knight to empower their rune weapon, effectively resetting the cooldowns on almost all their abilities and enabling many Runic Power abilities as well. Predictably, this ability is not affected by, nor does it trigger the global cooldown.
Curiously, this ability does not require a melee weapon, unlike its name suggests.
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This ability is seldom used in regular DPS or tanking rotations, but do not forget about it - it is very powerful and can turn the tide of a battle if used properly.