Merciless Resolve

• WWizards of the Coast2016-04-08
  Official Ruling
  You must sacrifice exactly one creature or land to cast Merciless Resolve. You can’t cast it without sacrificing a permanent, and you can’t sacrifice additional permanents.
• WWizards of the Coast2016-04-08
  Official Ruling
  Players can respond to this spell only after it’s been cast and all its costs have been paid. No one can try to destroy the creature or land you sacrificed to stop you from casting this spell or to make you sacrifice a different one.

Rulings via Scryfall