Speak with Lord Arkkoroc in Azshara. Begin your search at the waters edge to find the Temple of Arkkoran.
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.