The champion is Lady Liadrin , a Paladin whose hero ability is the same as Uther's: for two mana, you may summon a 1/1 Silver Hand Recruit.
The best news, however, is that while you will need to log in and play World of Warcraft to earn her, you do not have to pay any additional money -- even if you don't currently own or subscribe to WoW because it is free to play until level 20. According to the announcement:
"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 World of Warcraft account and play."
Current WoW players will have to grind a new character to earn the "Fledgling Hero of Warcraft" achievement, because it will not be rewarded retroactively to characters that surpassed level 20 prior to 11 a.m. PST on March 11.
Lady Liadrin was included in hearthstone patch 4.2.0, which was released March 14 in preparation for the new hearthstone expansion, "Whispers of the Old Gods." Full patch notes are available here.
This cross-promotion is probably an attempt to increase subscriber numbers as we head toward the anticipated 2016 release of World of Warcraft: Legion. An exact date has not been announced.
Players new to WoW who would like to unlock this hero may want to check out these leveling guides: