2016-01-22 : Activating the second activated ability after a creature has legally been declared as an attacker or blocker won’t change or undo that attack or block.

2016-01-22 : Endbringer untaps at the same time as the active player’s permanents. You can’t choose to not untap it at that time.

2016-01-22 : If an effect states that Endbringer doesn’t untap during your untap step, that effect won’t apply during another player’s untap step.