Rulings

Endbringer untaps at the same time as the active player's permanents. You can't choose to not untap it at that time.
If an effect states that Endbringer doesn't untap during your untap step, that effect won't apply during another player's untap step.
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.