Oracle Text

Untap Endbringer during each other player's untap step.

T: Endbringer deals 1 damage to any target.

C, T: Target creature can't attack or block this turn.

CC, T: Draw a card.

Card Rulings

1/22/2016 Endbringer untaps at the same time as the active player’s permanents. You can’t choose to not untap it at that time.
1/22/2016 If an effect states that Endbringer doesn’t untap during your untap step, that effect won’t apply during another player’s untap step.
1/22/2016 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.