All you need to do is level a character to 20 in World of Warcraft, and you’ll earn a new Achievement, Fledgling Hero of Warcraft, that will unlock Liadrin for use in Hearthstone. No World of Warcraft subscription is necessary to earn this hero and bring the power of the Light to bear in Hearthstone—simply log in to your existing account now and play.
Note that while this Achievement will not be rewarded retroactively, anyone who leveled a character to level 20 after 11 a.m. PST on March 11 will be eligible to receive Lady Liadrin.
Once earned, you’ll be able to find your new hero waiting for you in your Hearthstone collection (My Collection) under the Heroes tab.
Get on Your Hearthsteed and Ride
If you’re new to the fast-paced card-slinging action of Hearthstone, you can also earn a magnificent Hearthsteed to charge across the battlefields of Azeroth upon.
This epic flying mount is awash in a mystical blue glow—its rocky exterior just barely manages to contain the power that lurks within! When you’re ready to take to the skies, the Hearthsteed manifests ethereal blue wings, ready to carry you off to your next adventure.
Getting your hands on this unique flying mount is simple . . . and fun! Just win three games of Hearthstone in Play or Arena mode to earn the Mount Up! quest reward in Hearthstone. The next time you log in to World of Warcraft, the Hearthsteed will be waiting for you in your in-game mail.
Source : Become a Hero in Azeroth and Earn a Hero in Hearthstone!