Centaur's Herald

• WWizards of the Coast2012-10-01
  Official Ruling
  You can only sacrifice Centaur’s Herald once, so it’s not possible to activate its ability more than once.
• WWizards of the Coast2012-10-01
  Official Ruling
  Once you activate the ability of Centaur’s Herald, it’s too late for a player to respond by trying to destroy it or otherwise stop you from sacrificing it.

Rulings via Scryfall