After finding many people asking where the Endless Fleshfiend is for the Spying on the Endless quest in Freemarch, I decided to post a quick and dirty guide to finding it. I've helped a lot of people find it by personally taking them and helping kill the mob. But here's how to do it yourself.
The Endless Fleshfiend is a level 20 beast with a nasty AOE that reportedly does 370 hp per tick. So be sure to have a self heal available to you if you don't have a healer in the group. Better yet, bring a few friends lol
To find the Endless Fleshfiend, you have to make your way into the Iron Fortress – which is east of Meridian near Denegar's Stand. You are looking for the Endless Fiend in the northeastern quadrant of the Iron Fortress. When you come down the stairs in to the courtyard, you will be going right, past a few broken wagons toward the northern corner.
Once you get to that top right corner, you'll need to go up the incline on the left until you see the Endless Fleshfiend. Here's another pic and even a Youtube video I found on what the critter looks like.