If an effect states that Endbringer doesnâ€™t untap during your untap step, that effect wonâ€™t apply during another playerâ€™s untap step.
Endbringer untaps at the same time as the active playerâ€™s permanents. You canâ€™t choose to not untap it at that time.
Activating the second activated ability after a creature has legally been declared as an attacker or blocker wonâ€™t change or undo that attack or block.
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