Jötun Owl Keeper
Creature — Giant
Oracle Text
Cumulative upkeep
or 
(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.)
When this creature dies, create a 1/1 white Bird creature token with flying 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.”