Essence of Undeath

In preparation for and stocking up on this item for Legion, I have been trying the various locations recommended here. They may very well be good locations, however i found them highly populated and camped which in effect is not good for farming.

I decided to try Stratholme. From the main entrance I got an Essence of Undeath from one of my first pulls. Got excited and full cleared that entire Main Entrance area. Only the one.

After 25 clears, I only ever got 1 per run and it was always in the first small area of the zone. So I pared down my clears to cover from the entrance to the first gate you see on your right, then turning and clearing to the left down to a fountain. Somewhere in those mobs 1 essence drops. This is a very small area that takes 1 minute to clear, loot and leave.

In 25 short clears (1 minute turn around) I got 22 Essence of Undeath.

So it IS possible to not get one, and since this is just a small sampling I'm not sure what the real percentage will work out to be.

Just as a note, I did try the Service Entrance Area 10x thinking it would be a great place for farming since there are more mobs, and for many would do double duty if they don't have the Baron Rivendare Mount yet. But I got zero Essence from this section of Stratholme from my sampling.

So for me, it does seem the the small area I described above from the Main Entrance of Stratholme may be the quickest, and the most consistent reproducible farming of this item that I have found so far.

On the plus side no mob sharing, no respawn waiting, and consistent reproducible results.
On the negative side, the instance cap for an hour will slow you down.

The mageweave you get here may likely have more value in Legion since it is a requirement in making some of the tailoring cloaks that people may want for Transmog. Additionally there is a smattering of BoE that drop in the first area.

Happy Hunting and I look forward to feedback on this location.

Note: I thought i would add a section about getting to Stratholme which unless you are a DK is a total pain.
Alliance: If you have not completed the quest "The Battle for Andorhal" you can fly to Booty Bay and speak to NPC War-Mage Earlier (on the platform directly under Alliance FP) He will open a portal to Western Plaguelands. From there you can fly to The Plaguewood Tower FP in Eastern Plaguelands and from there go to Stratholme. Make sure to abandon this quest and it is repeatable indefinitely.
If you have completed the above quest, a long a** flight to Plaguewood Tower FP is in your future. (Eastern Kingdoms)

In WoD era, I would suggest going to Tyr's Hand in Eastern Plaguelands for this.
The Scarlet mobs have a decent 4-5% droprate, they're a bit spread out but there's several groups of two and the respawn is good.
I tried farming in Lake Kel'Theril in Winterspring, Noxious Glade and Corrin's Crossing in EPL, ended up with 2 TOTAL for half an hour each in all 3 spots.
This spot gave me 15-20 in 1.5 hours.

The downside is you can only farm there if you haven't done the quests for the area. Otherwise the Scarlet Crusade undead are chased out and you're stuck with friendly mobs...

Patch 4.3, I found that killing:
Anguished Highborne
Suffering Highborne
Wretched Spectre
Wretched Spirit
All in the area SW of Everlook (Winterspring) was by far the easiest way to farm Essence of Undeath.

With less than 2k health, these die easily and droprate averaged out around 2 essences per run. I rarely had to wait a full minute for respawns when circling the area.

Just avoid killing the blood elf ghosts, as they do not drop essences.

Essence of Undeath is still required by some professions to grind up tradeskilling, so alongside the cloth and coin found in this area, could be a good spot to farm some easy coin for lower levels.