On my server the Decomposing Ghouls actually give XP! This fact makes this Spot actually quite interesting if you are able to AOE such a Mass of Mobs down.
They can be found on the second Basement inside the "grove" in Wyrmskull Village near Utgarde, you can easylie get down there using XYZ.
Could give around 180k XP at once on Level 72, I just have no idea about the respawn rate.