Blizzard have clarified that the the Highland Drake: Embodiment of the Hellforged, the Aberus raid mount/Dragonriding customization, is in fact dropping from all difficulties, unlike what the Adventure Journal suggests.
Mount (Source)
We agree with a lot of the feedback around this.
The Highland Drake: Embodiment of the Hellforged dragonriding manuscript has a very low chance to drop from Sarkareth on Raid Finder, Normal, and Heroic raid difficulties.
Mythic raiders will always receive two manuscripts per Sarkareth kill until the end of the expansion.
