You may choose not to untap Tawnos's Coffin during your untap step.
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: Remove target creature and all Auras attached to it from the game. Note the number and kind of counters that were on that creature. When Tawnos's Coffin leaves play or becomes untapped, return the removed card to play under its owner's control tapped with the noted number and kind of counters on it, and if you do, return the removed Aura cards to play under their owner's control attached to that permanent.