For an expansion about the rising threat of demonic hordes, Legion has spent a lot of time focusing on other dangers.
Warlords of Draenor had several problems -- a lack of endgame content, a truncated storyline and a dearth of development where more than a year passed between the Hellfire raid and the next patch.
Legion may propose to be focused on the threat of the Burning Legion, but there are darker portents to be found, if one listens carefully.
The practice of magic is something that's always been in place in Warcraft.
Priests have been in the game since day one.