There is only one way to harm the demon lord and his minions. A weapon of power must be forged bearing his true name.
When such a weapon is forged, the demon and all under his command are vulnerable.
A lesser deity resides in Azshara: Lord Arkkoroc. You must seek him out but be warned, his minions will stop at nothing to destroy you if you are discovered.
Undoubtedly, you will have to pay a price for such information.
Completion
A demon lord?
<Lord Arkkoroc laughs.>
Perhaps the one can assist you with this information; but first, you must assist the one.
The temple is on the biggest island, in the northeast of the Bay of Storms, directly north from the edge of the southern "claw" of Azshara, just before you hit the northern "claw". The temple is filled with various semi-aquatic beasts (like murlocs). You can avoid a lot of them of them by hugging the walls (if you're 60). Arkkoroc is on the second floor inside.
Trivia
It may be related to the Christian Number of the Beast, which is the number of a great demon.