Intervene

• WWizards of the Coast2004-10-04
  Official Ruling
  If the spell’s target leaves the battlefield before this spell resolves, Intervene won’t resolve since it no longer targets a spell that targets a creature.
• WWizards of the Coast2004-10-04
  Official Ruling
  If a spell is modal, use the target(s) of the current mode only for this check. Targets for the other modes are never selected.
• WWizards of the Coast2004-10-04
  Official Ruling
  It means “targeting at least one creature” not “exactly one creature”.
• WWizards of the Coast2004-10-04
  Official Ruling
  Intervene can only target a spell that is currently targeting a creature.

Rulings via Scryfall