When The Dead Speak

The first time I did this, I thought the NPC on the highest mountain, with lots of lightening , and dragonkin, was the target.
IT IS NOT!
That NPC wil kill you quickly, and I took rezz sickness because I couldnt get up the mountain to recover my corpse.

You are looking for an NPC that is a tad NW of the highest peak.
He is in a clearing, with quillen

/way 32,3 9,6

Doing this quest as a level 90, fairly decently geared (so I thought) iLvl 492 fire mage, I found it very difficult to survive this fight. After you accept the quest, your Shado-Pan "sidekick" abandons you, leaving you to do this last Shado-Pan daily in the series solo.

The strategy that ultimately worked for me:

First, stand near the edge of the area and do your best to pick off the quillen one at a time. (This will aggro other quillen nearby, but it's not a problem as they have weak health pools and can be AoE'd.)

Then take care of any Beastmaster or Serpentbinder that may be nearby with his combat pet(s).

Finally, take on the Deathspeaker himself. He will summon gems; attack the gems. Stay out of the purple patterns on the ground which the Deathspeaker casts. Attack the Deathspeaker between your attacks on the gems.

For an elite, he goes down fairly quickly, and does only moderate damage on his own. As long as the trash is cleared out before the fight, and you attack the gems, it can be soloed even as a clothie. If I could do it, anyone can (fire mage being possibly the least survivable class/spec; cloth, no heals like a priest, and no pet like a frost mage or warlock).

EDIT: I did this again the next day on a different toon, and my Shado-Pan "sidekick" stayed with me for the quest, making it an absolute piece of cake. Not sure why she abandoned me when I did it the first time.