It is indeed quite obvious that HearthStone: Heroes of Warcraft is linked to Warcraft lore in more ways than one, but until now, there hasn't really been a direct connection between the two games in the sense of one requiring players to play the other. That has changed with the arrival of Hearthstone's latest hero, Lady Liadrin, who is essentially a re-skin of the game's default paladin, Uther. The interesting thing about Lady Liadrin is that she can only be had by reaching level 20 in World of Warcraft.
The tie-in was suspected by many when the latest patch of the game landed, and it is indeed here now. The requirement is specifically to "reach level 20" which means that those who already have WOW characters higher than level 20 do not actually receive the hero automatically. They too will have to start a new character and level it up to 20. Fortunately, this does not mean that those looking to grab up this hero have to buy a WOW subscription, since the game is free to play up to level 20 as it is.
While this move seems like a plot to get more people involved with WOW and ultimately to score more subscriptions, it is in fact in line with Blizzard's declared strategy of making the game more inviting to newcomers as well as to relapsed players. WOW's subscription numbers, which reached a 9-year low last November, are no longer reported.
The game is 12-years old after all and while it is more than natural for it now to bleed subscribers, it will get another massive lease on its life through the Legion expansion as well as through various promotions tied to the release of the Warcraft movie.
Philip Thalberg works for Gosugamers, THE destination for Dota 2 rankings.