2011-01-22 : 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.

2011-01-22 : You can cast Gravepurge with zero targets. If you do, you’ll draw a card as the spell resolves.

2011-01-22 : You choose the order of the cards you put on top of your library.

2015-02-25 : 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.