Some demons pledge allegiance to no one but themselves. A true enemy of demons finds and destroys his prey regardless of affiliation, however.
The demon Xeleth - a Legion defector - was once worshipped as a god by primitive swamp creatures in Zangarmarsh. When Draenor was nearly destroyed, he retreated into a slumber deep beneath Marshlight Lake. Find the portal at the lake and plant this spear into the ground.
Xeleth will awake - be ready to slay him and prove you're an enemy to all demons.