Soon after engaging him he'll boot you a far distance away and then follow. Downfall creates a pocket of air around himself that allows you to fly, but if you drop below or go too far away from him you'll fall. You'll then want to find Candrael, a Blood Elf Demon Hunter who stands on a hill above the western edge of the Hold (you'll want to take a route from the north to reach her.) She will then offer to toss you up in the air at the demon. While you'll be able to reach him if you jump down from other high points (such as Highmountain Peak) the official way to get to him is to find Candrael's charm, a random drop off of the demons in Felsoul Hold. There is a Fel Bat that flies above Felsoul Hold in Suramar called Downfall. Like the one for Truthguard above, this appearance is acquired by doing Withered Army training (see the above example for more.) There is no skull-face, and the default color is blue, rather than red. The Blood hidden appearance transforms your Maw of the Damned into a more one-sided axe, with the edge and the spiked pommel both showing the souls flowing into the axe.
Getting this appearance requires you to run the Withered Army training scenario for the Nightfallen and it is looted from the main chest at the end (the one you get no matter what.) While I think you may improve your odds if you score higher, it is ultimately a matter of luck (I did several full clears of the scenario with over 30 Withered left at the end - not that they account for that in your score - without getting it to drop.) Both the mace and much of the shield are made of purple (by default) crystals, though the Truthguard motif of the spinning central disk and the channels of light energy going right, left, and down are retained. The Protection hidden appearance is a Draenei-themed shield and transforms the sword Oathseeker into a crystalline mace. Note that alternate colors can be unlocked for each of these by doing 100 Dungeons using the appearance, completing 200 World Quests with the appearance, and killing 1000 enemies in PvP. While I know where some of the artifact appearances can be found for other specs, I figured I'd detail the ones I had found so far.