Gravepurge

• WWizards of the Coast2011-01-22
  Official Ruling
  You can cast Gravepurge with zero targets. If you do, you’ll draw a card as the spell resolves.
• WWizards of the Coast2011-01-22
  Official Ruling
  If you cast Gravepurge with at least one target and all of those targets become illegal, the spell won’t resolve and you won’t get to draw a card.
• WWizards of the Coast2011-01-22
  Official Ruling
  You choose the order of the cards you put on top of your library.
• WWizards of the Coast2015-02-25
  Official Ruling
  You can cast Gravepurge with no targets. If you do, you’ll just draw a card. However, if you cast it with at least one target and all of those targets become illegal, the spell won’t resolve and you won’t draw a card.

Rulings via Scryfall