Knochensplitter

Once you begin to cast Bone Shards, no player may take actions until you're done. Notably, opponents can't try to remove the creature you wish to sacrifice or make you discard your last card.

You must sacrifice exactly one creature or discard exactly one card to cast this spell; you can't cast it without sacrificing a creature or discarding a card, and you can't sacrifice additional creatures or discard additional cards.