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At the beginning of your upkeep, put a +1/+1 counter on Descendant of Masumaro for each card in your hand, then remove a +1/+1 counter from Descendant of Masumaro for each card in target opponent's hand.
All +1/+1 counters on Descendant of Masumaro are removed, including those put on it by other effects.
Descendant of Masumaro’s printed power and toughness is 1/1, so removing all the +1/+1 counters usually doesn’t reduce the creature’s toughness to 0.
The counters are added and then removed without a pause for priority or state-based actions being checked in between.
