Wall of Shards
Snow Creature — Wall
Oracle Text
Defender, flying Cumulative upkeep—An opponent gains 1 life. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)Rules for this card
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A keyword ability that imposes an increasing cost to keep a permanent on the battlefield. See rule 702.24, “Cumulative Upkeep.”
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A keyword ability that prohibits a creature from attacking. See rule 702.3, “Defender.”
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A keyword ability that restricts how a creature may be blocked. See rule 702.9, “Flying.”
Rulings (2)
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Wizards of the CoastWhen paying Wall of Shards’ cumulative upkeep, you may choose a different opponent for each age counter, or you can choose the same opponent multiple times.
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Wizards of the CoastPaying cumulative upkeep is always optional. If it’s not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can’t be made. For example, if a permanent with “cumulative upkeep
” has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay 
or sacrifice the permanent.