The Enigma Order

I was browsing wowhead when i came across this dagger:

Blade of Eternal Darkness

Binds when picked up
Unique
Main Hand    Dagger
33 - 70 Damage    Speed 1.50
(34.3 damage per second)
Durability 75 / 75
Requires Level 49
Equip: Chance on landing a direct damage spell to deal 100 Shadow damage and restore 100 mana to you.

This was written in one of the comments:

This blade has got NO internal cooldown at all, so IF you find yourself very low on mana and have the ability to use an AOE spell then you can get many times your mana back from downranking and switching to this blade in your main hand.

(Yes, as a lvl 70 char, have this myself and I used it with very great success on morogrim tidewalker on the murlocs when I was running low. I'm a mage and had only a resto shammy in my group so I had to do the most with what I had.)