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Thunderblade Charge deals 3 damage to any target.
Whenever one or more creatures you control deal combat damage to a player, if Thunderblade Charge is in your graveyard, you may pay {2}{R}{R}{R}. If you do, you may cast it without paying its mana cost.
Thunderblade Charge triggers only once per combat damage step per player no matter how many of your creatures deal combat damage to that player, how much damage is dealt, or who that player is. (It might be you if the combat damage is redirected.)
When the ability triggers, first you choose whether to pay {2}{R}{R}{R}. If you paid, then you choose whether to cast the spell.
