Template:Onyxia strategy
Onyxia is a powerful Legendary, which can create an army with a total of 14 Damage and 14 creature health from an empty board state, swarming your opponent with 1/1 Whelps. Each player can have a maximum of 7 minions on their side of the field, meaning Onyxia will summon up to 6 Whelps. Unusual for Battlecry summons, Onyxia potentially summons Whelps to both sides of herself, so check this effect to maximize positional cards like 
Dire Wolf Alpha properly. Note that if the battlefield is already full by the time her Battlecry is activated, no Whelps will be summoned. Note also that 
Brann Bronzebeard does not affect Onyxia, since the second battlecry fails to have any room to summon more minions.
Onyxia works especially well with druid decks, and to a lesser extent with shaman and paladin decks, and with effects which buff all of the player's minions. Synergistic druid cards include the card Soul of the Forest, which makes all friendly minions summon a 2/2 Treant when they die, AoE Buffs like Power of the Wild, 
Wisps of the Old Gods or 
Mark of the Lotus, as well as cards like Addled Grizzly, which will buff all whelps to 3/3's as they are summoned, generating quite a brutal army nearly instantly. Shaman cards include the card Bloodlust, which grants all friendly minions +3 Attack this turn, or Evolve, which can produce a great number of 2-cost minions (but beware of accidentally ending up with a Doomsayer). Paladin cards include Steward of Darkshire, which will grant all Whelps Divine Shield, and 
Sunkeeper Tarim if Onyxia is killed but Whelps remain, which will instantly transform them into 3/3's.
Many neutral minions are powerful with her as well - if you can combo the whelps with either a Raid Leader, a Dire Wolf Alpha, or Stormwind Champion, you can get a lot more out of them than what you can expect from six 1/1's. (Remember to place the Dire Wolf optimally so that it affects the maximum number of Whelps.) Knife Juggler also works well with Onyxia, dealing 1 damage for each Whelp summoned, as well as for Onyxia herself, for up to 6 damage if it is the only card on the board when she is summoned. 
Mukla's Champion can be very powerful, buffing the army of whelps every turn, resulting in quite a snowballing army. Cult Master may be among the most powerful minion cards to combo with Onyxia, as if it is played the next turn, the player can draw up to five cards from sacrificing Whelps attacking enemy minions. (One Whelp will have to go away first to make room for the Master if the board is still full.) 
Emperor Thaurissan, like Aviana, can increase the power of her army even further by reducing her mana cost and allowing the player to cast buffing cards on the Whelp army the turn that she is played.
The biggest dangers to Onyxia's dominance are board-clear cards like Twisting Nether, Swipe, Lightning Storm, Consecration, Meteor, Flamestrike, Blizzard, and even the humble Arcane Explosion and Fan of Knives. DOOM! in particular turns her boardstate into an incredible advantage for the opposing player, obliterating her army while simultaneously more or less filling the enemy player's hand.
As such, Onyxia is best used as a late-game finisher when your opponent has already used up their board clearing cards and has few or no cards left in hand.
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