Altruis the Sufferer wants you to take the Imbued Silver Spear and use it at Portal Clearing near Marshlight Lake in Zangarmarsh to awake Xeleth. Return to Altruis after you've slain the demon.
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.