Meet the following conditions to unlock the following card backs.

Card Back	How to unlock
Blizzard 2014	Promotional reward for attending or for purchasing a Virtual Ticket to BlizzCon 2014.
Blizzard 2015	Promotional reward for attending or for purchasing a Virtual Ticket to BlizzCon 2015.
Dalaran Flame	Promotional reward for active TeSPA (The eSports Assosciation) members belonging to a local chapter with at least 25 members.
Fireside	Play 3 matches against other players on the same subnet (local network), while at least 3 players are signed in on that subnet.
Galaxy Gifts	Unlock the "Galaxy Gifts" achievement. (See the "Unlockables" section for more details.)
Golden Celebration	Promotional reward for placing in the top 128 of certain tournaments.
H01: Alleria	Purchase the "Alleria Windrunner" Hero.
H02: Magni	Purchase the "Magni Bronzebeard" Hero.
H03: Medivh	Purchase the "Medivh" Hero.
Heroes of the Storm	On the same Battle.Net account, reach Player Level 12 in the game "Heroes of the Storm".
Heroic Naxxramas	Defeat all 15 Curse of Naxxramas bosses in Heroic Mode.
Legacy of the Void	Promotional reward for pre-purchasing "Starcraft II: Legacy of the Void" Digital Deluxe Edition or Collector's Edition on the same Battle.Net account.
Legends	Reach Legend rank in Ranked Play mode in any season.
Molten Core	Pre-purchased the Blackrock Mountain adventure using real money before its release.
Nefarian	Defeat all 17 Blackrock Mountain bosses in Heroic Mode.
Power Core	Promotional reward for attending the "2015 EUROPEAN ROAD TO BLIZZCON" event in person.
S01: Pandaria	Reach Rank 20 in Ranked Play mode in April 2014.
S02: Black Temple	Reach Rank 20 in Ranked Play mode in May 2014.
S03: Rainbow!	Reach Rank 20 in Ranked Play mode in June 2014.
S04: Naxxramas	Reach Rank 20 in Ranked Play mode in July 2014.
S05: Icecrown	Reach Rank 20 in Ranked Play mode in August 2014.
S06: Pirates!	Reach Rank 20 in Ranked Play mode in September 2014.
S07: Hallow's End	Reach Rank 20 in Ranked Play mode in October 2014.
S08: Goblins	Reach Rank 20 in Ranked Play mode in November 2014.
S09: Gnomes	Reach Rank 20 in Ranked Play mode in December 2014.
S10: Maraad	Reach Rank 20 in Ranked Play mode in January 2015.
S11: Lunar New Year	Reach Rank 20 in Ranked Play mode in February 2015.
S12: Ragnaros	Reach Rank 20 in Ranked Play mode in March 2015.
S13: Cupcake	Reach Rank 20 in Ranked Play mode in April 2015.
S14: Ninja	Reach Rank 20 in Ranked Play mode in May 2015.
S15: Darnassus	Reach Rank 20 in Ranked Play mode in June 2015.
S16: Darkspear	Reach Rank 20 in Ranked Play mode in July 2015.
S17: Tournament Grounds	Reach Rank 20 in Ranked Play mode in August 2015.
S18: Exodar	Reach Rank 20 in Ranked Play mode in September 2015.
S19: Highmaul	Reach Rank 20 in Ranked Play mode in October 2015.
S20: Explorer's Map	Reach Rank 20 in Ranked Play mode in November 2015.
S21: Shaman Thrall	Reach Rank 20 in Ranked Play mode in December 2015.
S22: Love is in the Air	Reach Rank 20 in Ranked Play mode in January 2016.
S23: Tauren Thunderbluff	Reach Rank 20 in Ranked Play mode in February 2016.
Staff of Origination	Clear all 13 The League of Explorers encounters in Heroic Mode.
The Grand Tournament	Pre-purchased a 50-pack bundle of The Grand Tournament set using real money before its release.
Warlords	Promotional reward for purchasing World of Warcraft: Warlords of Draenor Collector's Edition or Digital Deluxe Edition.
Winter Veil Wreath	Win a game in the "Gift Exchange" Tavern Brawl in December 2015.