Dark Salvation

• WWizards of the Coast2016-07-13
  Official Ruling
  You may cast Dark Salvation with X as 0 just to have a creature get -1/-1 for each Zombie a player controls, and you may cast it without targeting any creature just to have a player get Zombie tokens. If you really just want to cast a spell, you can even cast it with X as 0 without a target creature and have neither effect do anything.
• WWizards of the Coast2016-07-13
  Official Ruling
  You choose up to one target creature total, not a separate target for each Zombie that player controls.
• WWizards of the Coast2016-07-13
  Official Ruling
  If the targeted creature becomes an illegal target, the targeted player will still get Zombie tokens.

Rulings via Scryfall