Players who earn the achievement will receive Lady Liadrin, the first Blood Elf Paladin, the proof that you can rip away the power of a Naaru for months but still be totally redeemed as a for-real paladin by apologizing really sincerely, and the seven-time pie-eating champion of Silvermoon City. There’s also still a Hearthsteed available for players who win three matches in Hearthstone and then log in to World of Warcraft, if you don’t already have that cross-promotion.
World of Warcraft, you have probably taken Hearthstone for a spin. The game is now offering something extra to entice you if you are a WoW veteran: you can earn a new hero in Hearthstone! Admittedly, you have to bring a character to level 20 after March 11th, but odds are you can manage that in about an hour of play, maybe two if you really pace yourself.
Source: Official Site