Killing any non-trivial demon in Krokuun, Mac'Aree, and the Antoran Wastes in Argus helps you work towards a set of achievements that ultimately requires you to kill a total of 5,000 of these. For killing 2,000 you get awarded with "...And Chew Mana Buns" achievement that gives you a mount called Bleakhoof Ruinstrider. At 5,000 demons you get "And We're All Out of Mana Buns" and the title Demonslayer, so it's worth farming it!
Slaying a total of 5,000 might seem like something that takes a long time, but lucikly it's really easy and fast if you use a certain method. What you need to do is go to Krokuun 60.00, 78.00 (/way Krokuun 60.00, 78.00) and kill the demons there. Stand close to the two green portals there, and kill any demon that comes out from it.
You can easily solo farm 1,000+ per hour, but if you want it even faster open up Premade Groups and look for groups for this. You can do it in a raid group with 40 people, and you don't even need to hit each demon to get the credit! With 40 people at this spot getting 5,000 demons will be very fast, and done in around an hour or so.
Once you're done head over to a mailbox where your brand new mount os waiting for you!