You may move +1/+1 counters from any number of other permanents you control, not just from one.
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wotc
You may choose to move zero +1/+1 counters.
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wotc
If a creature has been dealt damage, that damage remains marked on it until the cleanup step. If Aetherborn Marauder takes counters from a creature that has been dealt damage, that creature will be destroyed if the damage is now lethal.
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wotc
You choose which counters to move as Aetherborn Marauder's triggered ability resolves.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.