Novablast Wurm

• WWizards of the Coast2010-03-01
  Official Ruling
  Novablast Wurm’s ability triggers and resolves during the declare attackers step, before blockers can be declared.
• WWizards of the Coast2010-03-01
  Official Ruling
  Novablast Wurm’s ability destroys all players’ creatures (except Novablast Wurm), including yours.
• WWizards of the Coast2010-03-01
  Official Ruling
  If two Novablast Wurms attack, they’ll destroy each other. This happens before they can deal combat damage.

Rulings via Scryfall