Viscera Seer

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2010-08-15
  Official Ruling
  If you sacrifice an attacking or blocking creature during the declare blockers step, it won't deal combat damage. If you wait until the combat damage step, but that creature is dealt lethal damage, it'll be destroyed before you get a chance to sacrifice it.
• WWizards of the Coast2010-08-15
  Official Ruling
  You can sacrifice Viscera Seer to activate its own ability.

Rulings via Scryfall