Anowon, the Ruin Sage

• WWizards of the Coast2010-03-01
  Official Ruling
  When the triggered ability resolves, first you choose which creature you'll sacrifice (if you control any non-Vampire creatures), then each other player in turn order does the same, then all chosen creatures are sacrificed at the same time.
• WWizards of the Coast2010-03-01
  Official Ruling
  If a player controls no creatures, or if all creatures a player controls are Vampires, that player simply doesn't sacrifice anything.

Rulings via Scryfall