Ice Block is an epic mage secret card. Ice Block can be obtained through card packs, through crafting, or as an arena reward.
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Can be very effective as an endgame surprise and can be a clutch game saver, giving you one more guaranteed turn.
Note that Ice Block will be triggered in response to any attack (during the opponent’s turn) that would deal fatal damage to the mage – the mage’s health will then by frozen at its current value. For this reason it is generally desirable for opponents to attempt to get the mage’s health as low as possible before triggering damage. In this way, when the mage loses Immune at the start of the next turn, they will have a minimum of health remaining.
More at: http://hearthstone.gamepedia.com/Ice_block